I’d have thought more people would have commented on this, so maybe it’s just me. IMHO, this is a huge mistake. Bad habits are as easy to form as they are hard to break. Learning things correctly from the get-go will save you countless hours of un- and re-learning. Even if you take a few introductory lessons you’ll be miles ahead. Going back for monthly or bi-monthly check-ins/ups will do you a world of good until you can start taking them regularly.
It depends on what it is, and what you want to play on it. I am using a Yamaha CP-300 for a couple more months while I’m on the road. It’s okay, but it isn’t even as inspiring to me as an upright, and I miss my grand piano something fierce. The action on the Yamaha is pretty good, but the sound isn’t quite right, especially when you use the sustain pedal. To me, there’s nothing like the sound of a real acoustic instrument.
Then again, it’s better than a piece of wood with keys painted on it…
I’ve never played piano, but played guitar around 35 years. Two things to consider (which was offered to me by a friend who plays piano): whether you are interested in performing unaccompanied, and whether you are interested in singing. If your answers to these are ‘yes’ and ‘no’ respectively, piano may be the better choice.
The reason is that playing guitar unaccompanied is more likely to require singing, whereas solo piano playing can stand on its own better.
There certainly are solo instrumental guitar styles (as demonstrated by artists like Leo Kottke, John Fahey, and classical players). But for most people, it will take longer to learn to play them than it will to play piano well enough to perform solo.
Such concerns about guitar are lessened if you want to play solo and also want to sing.
As a counterexample, I started piano lessons at six. I stopped the lessons after three or so years but kept on playing by ear, but learned to read well enough that, by high school and college, I became a pretty good ragtime pianist. The problem was, I could read only if I knew how the music went in the first place, and, furthermore, couldn’t manage to branch out into other styles of music. For instance, I couldn’t handle jazz or classical, and I was getting bored with ragtime.
During my junior year in college, I took up the guitar in earnest. A friend showed me the basic barre chords and “boxes” used in blues and early rock and roll, and I never looked back. I started playing classical guitar as well, and my sight reading got much better. Why that happened I really don’t know, unless it has something to do with the differences in how we use our right and left hands on guitar vs piano. As for musical notation, tab is often preferred by guitarists, for the obvious reason that so many of them can’t read music; but I would rather read something in standard notation, with at least a few fingering notes, than tablature which is nothing but fingering notes.
I sight read fairly well, but my preference for guitar is tab with rhythm markers. Why that isn’t becoming more standard for guitar is beyond me. If I write for the guitar that is always how I write. It’s much more straightforward in the same way that standard notation is very straightforward for piano.
Background: I learned to play the piano as a child and I’m trying to learn to play the guitar.
To me, the piano is way more intuitive and much more logical than the guitar. And melodies are nice, you don’t seem to get them as easily (or usefully) with a guitar.
To add to my post above, here’s one reason why I’m playing guitar again, beyond simple inspiration: I find guitar much more expressive than piano. On piano, you control the weight that you strike a key, plus pedals (the sustain pedal being by far the most useful/used one). Which is all very cool, but that’s the limit of your control over the tone of the instrument.
On the other hand, on guitar you can bend notes, do slides/glissando easily, add pinch harmonics, and other flairs. You can add some percussive elements (chucka-chucka). On electric guitar, you have available a huge range of equipment that changes the tone of the instrument, subtly or extremely.
The pure sound of a piano is very compelling to me, but guitar is just more interesting, flexible and fun (to me).
My main gripe with guitar, and the reason I don’t play it more often, is that I get frustrated with its intonation. I have a nice acoustic guitar and there is nothing wrong with the intonation it just has the standard guitar problem where you can’t get all open and barre chords sounding equally in tune. The odd thing is that I notice it clearly when playing, but if I listen to a recording of myself it is much less obvious, I think I’m picking up the beat frequency through my hands or something.
Even when it is nicely in tune, I notice the equal temperament compromises much more. The slightly out of tune sound of a major third sticks out like dogs balls to me, but only when I’m playing it myself.
Hmmmm, this reminds me of the customer in my boyfriend’s music shop who asked:
“I want to buy an accordeon or a concertina*, which do you recommend?”. 
Surely, you’d want to play the instrument that you like most, that speaks to you most. I can’t imagine choosing an instrument on any other criteria, except maybe financial constraints. Otherwise, if it is heavy, loud, difficult, hated by most of the world or boringly common: If you love it, play it. Learning to play an instrument, for most people, involves a fair amount of work and discipline so that if you don’t feel strongly about your instrument, you’re likely to give up.
*A concertina is a small reed instrument used in, among other things, Irish traditional music.
I go with everybody else that says "whichever you like best/feel more comfortable with.
That said, piano all the way! I’ve just started playing it an year ago and I’m having tons of fun. To me it’s a much more versatile instrument and much easier to play because it’s so much more logically organized than a guitar. I’ll never be good enough on a guitar to play the music I’d really like to, but on the piano I feel I have a fighting chance. You have to be pretty good to play classical or jazz guitar, but you can be a mediocre or weak jazz or classical pianist and still play. Besides, you can still play rock or pop or blues or almost anything you like. As I’m doing it for fun and for myself and don’t mind if I’m never good enough to play for/with other people that seems a better deal to me.
However, I found reading music to be relatively easy, so I don’t know that this should factor so much in your decision. Maybe it’s because I learned to do it on the piano, but the whole thing seems to me to be so simple that I think maybe people who have difficulty at it make it harder on themselves by believing it to be more complicated than it is. (I’m not bragging. I suck at music in general and have no innate talent at all. Reading sheet music is maybe the only thing I’m any good at, so I figure that if I can learn it people who have a tough going must be making a mistake somewhere.)
I wouldn’t say that this by any measure is a “standard” guitar problem. If it really is a “nice” guitar you owe it to yourself (if you want to keep playing it) to take it to a shop or luthier and have it properly set up. The neck torsion can be adjusted, or if necessary the bridge and/or nut in ways that greatly affect the action and intonation. A guitar should not have intonation problems. If you ever changed string gauges, for example, the neck torsion may need a slight adjustment to correct the intonation. It’s possible to do this yourself if you know what you’re doing and have thoroughly researched it, or it’s usually a simple enough operation for a professional.
It would suck to be discouraged from doing something you like by such an easily correctable* frustration.
*correctable if your guitar really is decent. If it’s a 10 year old $60 Wal-Mart guitar or something, it might NOT be correctable.
It’s a $1000 guitar and the intonation is fine, it’s just a side effect of the way fretting strings work. The best guitar in the world will not be in tune everywhere along the neck. The problem is that I can hear it when I play it, and as I said, I can’t hear it when I’m listening to a recording of me playing it and I can’t hear it when I’m listening to someone else play it, so it seems to be something I’m particularly sensitive to when I’m playing it.
I learned to play piano fairly well when I was 14 years old; mother made me do it, but I hated the recitals and quit. Many, many years later, I decided to take up the guitar. Knowing nothing about it, I went to a store that specialized in guitars and was advised to buy a classical guitar, which I did. I have short, spatulate fingers, and found the wider fret board difficult. Because of my piano training, I didn’t have much trouble reading music. I took lessons and eventually was able to do many classical pieces. Decided to try an electric guitar; lot of trouble transitioning from a classical to electric. I gave up on it. Since then, I’ve been thinking about piano again, but haven’t made a decision about it. Bottom line: looking back, I’d definitely say that piano was easiest. But as others have said, what is your goal? Do you want to play for yourself and family and friends? Or do you have thoughts of joining a group and performing in other venues?
Guitar, only because it’s a very social thing to have. Good for parties and other gatherings, etc.
Oh, definitely. You need to go the other direction entirely. Get the lessons first, and after a while you can move more towards being self taught.
I mean, I mostly play by ear, and taught myself how to do that. But I think the main reason that was so easy for me was that I got basic piano lessons when I was six.