Choose the side you enjoy.
If the 5 other threads which have devolved into flame wars and petty arguments aren't proof enough, you won't get an unbiased, helpful answer here. You pick the side you enjoy more, and don't let faction fanboys or the "I've been on both sides and <faction> is so much better!!!" influence your decision with fallacies and soothing words. Nobody here knows what side you should be on; only you should decide.
There's been at least 5 threads in this section on who is better in PvP, and quite frankly the only useful information you'll get out of those threads is the fact that each side talks a lot of crap. You're going to hear the entirety of WOHL or other gungho Horde players claim their dominance, and then see a rebuttal by angered Alliance players, and thus the flame war will begin.
So please, before you or anyone else starts another thread like this which will almost certainly do nothing but instigate a bragging/flaming/bashing war, just pick the side you enjoy. Of particular interest would be a guild; find one you like and stick with it.
Premades in general don't lose often. And when they clash, you can ask Jyxi, they produce (often) mixed results.
Search "Contemplating going to the Horde" in this section. There's your predecessor and how it turned out.