Lets look at the option:

10 Warlocks in non-rated BGs = 10 extra pets, Lots of AOE fear, if demonology then impswarm plus potentially another big pet for 20 seconds + doom guards ... so let me count 30 pets, plus imp swarm x 10 - WSG lags out and the opposing team is like WTF...

10 Boomies = 30 trees, 10 beams of AOE silence so you can always nuke healers, dots, massive AOE capability potentially on round robin ... roots, speed of play (travel form, the stampeeding roar thing, plus the ability to stealth, and all the other non-movent based CC capabilites druid are pseudo-immune to). Boomkins depending on how they are played are either bad knock offs of mages or locks depending on if you like wrath spam or if you are into the who spreading dots everywhere before going casted spells.

I could see either being good. But 10 boomkin = able to do LFD in groups of 4/5 where as the locks have to do in groups of 3 if you are in to PVE. If this is just PVP both are pretty spectacular.