The PvP is even when it comes to BG's and WPvP. Depends on the team comp and times of the day. As far as WG goes....I wouldn't say the horde have it 80% of the time. I would say the horde uses more multiboxers in WG. If there were some rules against the amount of multiboxers used, it would be a much different story. Again, WG depends on the time of the day. There is not a more dominate side or best pvp guild out there. Some guilds will take anyone with any gearscore just to have the numbers in a fight. Or as I like to call them.....meat shields for the good pvpers.
Just pick a faction. In the end, it really doesn't matter. Be the underdog. It's more fun anyways. Don't just pick a side because you think they win all the time. What's that say about you??
Mercenary mode is random. Sometimes you get selected, sometimes you don't. I had days where I was the opposite faction all day, then other days where I never joined the opposite side.
I know this isn't helpful, so I guess I'm telling you to play what you want to play. Don't worry who is doing what, and who is winning today, cause tomorrow it will change.