This is not a who would win in a fight thread, this is my opinion on who is better.
I believe the old gods are a way better super villian than sargeras, think about it how do the burning legion try and take down the allianc/horde, they roll in with there armies and try to fuck shit up. sort of a rine and repeat battle tactic.
The old gods on the other hands corrupt, Yogg corrupted all the watchers, not a simple thing to do, some old god corrupted Deathwing, a freaking aspect. they have the ability to make a MUCH more interesting storyline than the legion. imagine if when Cata hits and you learn and old god has currupted Garrosh and he is now leading the armies. a complete twist that would never happen, but it "could" happen simple because the old gods can corrupt anything/anyone.
These are just my opinions, and i thought i would share my train of throught.
TL;DR Sargeras = Massive Armies < Old Gods = Corruption