This is a personal choice. What kind of music do you like? What do you want to accomplish from your music lessons? Do you want to play for or with others? Do you want to write songs?
I play guitar, and I’ve taken piano lessons as an adult. They both have their strong points. If you like classical music or Elton-John style singer/songwriter stuff, you might want to play piano. If you like Rock, Folk, or coutnry, you might like to learn guitar.
If you want to play with others, go for the guitar. It’s a lot easier to find somewhere to play if you’re bringing a guitar with you. You can find lots of amateur adult musicians who like to get together and jam informally. There are open stage nights in bars and lounges. You can take a guitar with you camping and play in front of the fire.
When do you like to practice? A piano is a pretty loud instrument (unless you have a digital piano with a headphone jack). You might find that your playing is constrained by the times in which you’re not disturbing others.
One thing about guitar which has always caused me to put it aside for longer periods than I want - you have to build up calluses on your frettnig fingers, or it rapidly gets painful and more difficult to play. If you stop playing for a while, the calluses go away and then you have to spend time building them up again. Also, at least for me I have to keep the nails of my fret hand very short, which means I have to prepare to play, which inhibits that spontaneous late-night pick up the guitar and play a tune thing.
Guitar is going to be easier to get to a point where you can play a song in a way that sounds competent. You’ll spend a lot of time at the piano playing scales and songs like “Camptown Races” before you can play much that sounds current and complete. But on guitar, if you can learn the C, G, D, E and Am chords, you can strum along with an awful lot of popular songs. However, once you get past that point, the difficulty level goes up and guitar gets to be as hard as piano.
There are other instruments you might consider. If you played trumpet, you might consider something like an Akai EWI. The USB model plugs into your computer. It’s a professional grade instrument that can be played like a sax, a clarinet, or even as a trumpet (it has a setting for playing brass valve fingerings). You can have a lot of fun with this thing, because it’s connected to your computer. So you can do multi-track recording, you can get play-along tracks for it, you can make it sound like anything with software synths, etc. I have an EWI 4000s, which is the higher-end model with built-in synthesizer, but they play pretty much identically. I absolutely love mine. I play it pretty much every day.
So really, it comes down to a lot of personal details that only you can decide.