I had a similar experience, but I started on Whisperwind (PvE) as alliance. Horde was outnumbered, but not by a terrible amount. Battles at Crossroads and TM would go back and forth most of the time. Not sure if other servers did this or not, but before people on my server decided TM/SS was better, Crossroads was the place to gain honor in large battles for a few weeks. Later I moved to one of the PvP servers and rolled some horde characters. We definitely outnumbered the alliance as I barely saw any of them in the world and our queue times for BGs were absolute hell. Eventually cross-realm BGs happened and things got significantly better.
From what I've read it seems like more people want to go horde on PvP servers due mainly to racials for the PvP benefit, and of course because horde has undead and orcs instead of fleshies and elves. Most of the top PvP streamers are horde, and most of the people in chat seem to have a hardon for horde. Who knows, maybe the alliance people are just quieter. I played both sides on the stress test, and I will say the horde chat was much more childish yet entertaining in true barren's chat tradition. Anyway, this is all just a guess on my part based on not much. As most of the guild seems to want to roll PvP, you can safely ignore the alliance bonus population that PvE servers are said to have.
When BGs come out we're getting the cross-realm queuing, so population amounts really only matter in the open world. Pre-BG honor grind and wPvP, grouping for content, leveling in the world without being perma-ganked, getting into raids without being counter-raid cockblocked, etc. From what I've seen of world chat, avoiding streamers seems to be a decent idea unless you like seeing endless spam about them for hours and hours. Either way, it's gonna be fun and I can't wait until release.