First of all, I have both lvl 80 characters on Horde and Alliance. The thing is, that I want to level all classes now on one faction only. I really enjoy dungeon leveling and one thing I noticed is that on EU times it's easier to join a RDF on Alliance than it is on Horde specially for DPS classes.
I think it's mainly because I play DPS, but I also think it's because the amount of paladins (who can play all 3 classes pretty decently) that Alliance has the advantage on RDF's queue timers for a dps player. On Horde leveling there are less paladins and warriors don't do very well.
So, as a dungeon leveling lover and someone who wants to invest everything on one faction. Should I go for Alliance or Horde?
Low level RDF could take up to 1 hour in prime times (EU evenings)
High Level is usually max 10-20 minutes (for me its mostly 1-2 minutes after queue) again in prime times