If you prefer PvE and want to play with humanoid characters, then choose alliance.
If you prefer PvP, want to be aggressive because you are the Big Bad horde and want to play as a monster (that doesn't apply to blood elf characters, but even then you must play together with monsters, if you are in party/raid), then choose horde. Choosing a tauren character will guarantee, that every alliance player will laugh at you (it's pretty funny to see talking/fighting cows).
It's a fact, that more kids play alliance, than horde. But that doesn't mean, that alliance is full of kids. For example on our server (Bloodhoof EU) alliance is always the first in finishing new instances. I'm 100% sure, that the first player on our server with full T4 and T5 will be alliance
I'm sure, that with the expansion many kids will start a BE character (mostly the alliance noobs without a guild, or with a noob guild which can't kill anything in molten core), so horde will fall back in PvP and horde general chats will be full of spams
