I ride both! I started out riding English then switched to western for a time and I've been like a ping-pong ball going back and forth ever since! I like them both for different reasons.
I really haven't done much competition wise except on the Intercollegiate riding team while I was going to school. I didn't have a lot of opportunity as horseless kid unless I could bum rides off my friends'
I ride western horsemanship and pleasure style (no speed events for me!), and hunters (on the flat because I'm to much of a chicken to jump). And I'm sure some of the other English folks out there will argue that stock horse hunters are basically just western horses being ridden in an English saddle, although I've also started up with dressage as well.
Anyway when you get right down to the basics of it they really aren't all that different other than the tack and specialized events you can go into.