Alliance is usually more casual players, you know, mostly 14-17 year olds, just playing this in their spare time. Horde is more of the 21+, read all of the books, knows how to make a private server, plays for over 24 hours on end eating doritos and drinking mountain dew from a 2 liter bottle than peeing into said bottle so they don't need to get up, openly devoting their exsistence to the game kind of person. Of course, either way it's just a stereotype. The real question is what race you like most. You want to play an undead? Don't let the enormous NEEEEEEERD population keep you away. There are plenty of casual players there too. Just not as many.
Those must be bum 21+ year olds... No job, eh?
It all depends on what you like. From the first few replies or so that I read, it looks like everyone is being biased. They forget to mention that it all depends on the server you choose too. Some servers have "more mature" people playing horde, others have "more mature" people playing alliance.
As I've played both, I've noticed that it's all the same, no matter which faction you choose. You always run across that one person that is immature and thinks they're "all that", and then there's the people you run into that are awesome to play with.
In the end, it all depends on your preference. What you like (or what looks you like), etc.