I will let you in on a little secret. They are exactly the same when it comes to the people who play either faction. As Bon Jovi once said, only the names will change.
Different servers have different population balances, but they still manage to get your run of the mill trolls and such on each of them. Only they have different names than the ones on your other servers.
Horde is not more mature than Alliance. Alliance is not more mature than Horde. They both have about the same number of idiots. The idiots are the ones who fill up much of trade chat. You have people making Anal jokes on every server. There is Chuck Norris chat in Barrens on every server. There are people spamming WTB stacks of Eternal blah blah, Frozen Orbs, hibbity bibitty for X gold per stack.
There are Ninjas. There are trolls. There are morons. They are all available to you regardless of what server you are on.
If someone asks a question about their class in trade chat such as "What is better for a Ret Paladin, strength or crit?", no matter the server you will get people answering "Spirit" or MP5.
So, if you think the grass is greener on the other side, think again. If you plan on faction or server transfering, you should look at the reasons for doing so.
1) Because of friends on another server?
2) Looking for better PvE progression?
3) Want to go from PvP to PvE or vice versa?
4) Want to not look stupid like every race on your faction?
These are some of the legit reasons for wanting to faction or server change. The one that will be a big waste of money is if you are looking for the "more mature" players. You can get that in a guild, but not on a server as a whole.
If you like Horde, play Horde. If you like Alliance, play Alliance. Don't think the grass is going to be greener, because you will run into the same shitheads on both sides.