I guess I would choose
Kristen if I had to.
Kirsten looks like a mistake, and so many people pronounce it Curse-Ten, which I think is very unattractive.
In fact there is a little girl at my daughter's school whose name is Kurstyn. The poor child is like a cartoon character, with this round pasty face and big freckles and frizzy red hair that's almost orange. I've never seen hair like that on a little kid.
I think
Kirsten might be more popular now, at least for younger kids. I know of this Kurstyn and I think a friend of a friend has a
Kirsten who's about ten, but I don't know of any Kristens that young.
Kristen seems to have been a big hit in the late seventies and early eighties and then went downhill fast.
I sort of think of
Kristen as a bimbo name, but the kind of bimbo who is also an overachiever, if that's possible. I went to school with several Kristens and a couple of them were like that.
I think of the names as very feminine, because I've never heard of a man being called either one. But I like
Christina or
Kristina much better.