Alan as a first name, Allen as a last name.
ETA: The name Morgan also works as a first or last name. Or male or female first name.
I have a cousin named Alan and a cousin named Allan. My son’s middle name is Alan after his dad’s father. I would default to the Alan spelling if I didn’t know for sure.
I have an uncle on either side, both named Alan. Allen is nice as a last name, but as a first name it makes me think of that dorky boy from 3rd grade. Sorry, Allens.
I hav encountered many variations on “Alan.”
My experience has been that “Alan” is the most common spelling when we’re dealing with first names, while “Allen” is far more common when we’re dealing with last names.
Remember, it’s Alan Shepard, Alan Arkin, Alan Ladd…
But it’s Ethan Allen, Steve Allen, and Ray Allen…
What SciFiSam said: the Welsh spelling, Alun, of the people I know. (In Welsh, allan is a common word meaning “out,” and allen more or less means “could.”) Of the people I’ve heard of (i.e. famous people), and the spelling I prefer, Alan.
I’ve known only one Allyn in my life, and she was a girl. I guess she actually about 53 now. She had sisters named Sidney and Leslie, so I think her parents just had a thing for gender neutral names.
Really, not Allan? Not sure what universe I’m living in.
How many Allans can you name with that as their first name? Better still, how many of these do you even recognize?
Just askin’. 
There’s an English footballer called Gary Neville. His middle name is Neville, after his dad, Neville Neville.
Allan - because that’s how my brother’s name is spelled.
You scare me.
I think “Alan” is the only one I’ve ever come across among people I know. And it’s what I think of as the default spelling for a first name.
Alan.
My first ex-husband was an Allen, though.
ETA - Allyn looks like a feminized version of Alan.
Allan Morris is a friend of mine (since we were at university together), but I also had an Uncle Alan, who is now dead, so I feel that “Alan” is the more normal spelling.
All my names (first, middle and last) are about equally common as first and last names. (I won’t say what they all are, since I want to preserve a small amount of anonymity here.)
Never seen Allyn - did you mean Alleyn which is a well-know surname?
But the two Gonzalez are lastnames.
Allen is right
surprised to see so many wrong answers in the poll
Huh, few. I guess between Edgar Allan Poe and that Allan doll I had as a kid, Allan has been the only way for me. They got to me early.
This is exactly what I was going to say.