Arenas pop mostly all the time tho sometimes slow queues due to only few teams queuing depending on the time / day. Bg's pop everyday atleast once i think and every class can global anything from 100-0 in 3 secs.
I've won the vast majority of my BGs, and I'm on Alliance. I don't know if it's just me getting lucky with teams or if Alliance>Horde in battlegrounds at 70. Would have to ask some more people.