So now that we've established that haste belt is pointless for blm since no mob lives long enough, how about we go back to the original question of which is the overall best belt?
Personally, I'd go for cognition (but I still use penitents <_<) as it's going to give you
consistantly higher damage (*** all difference though), rather than the occasional damage boost. Also if you use the old estimate of 2 INT = 1 MAB, cognition still wins out. Witch sash has a 2% chance of giving you 10 MAB, which averages out to .2 MAB per nuke. There's the other issue too, where if you work out you can kill a mob with a Blizz V and a Fire V, then the occasional damage boost will be wasted, because you're still going to be casting those two spells. Then again, that's probably something that applies more to soloing flans at 75.
I've looked into blm a fair bit over the years, but only just leveled it a few weeks ago, so my opinion comes more from old readings than real experience. Just keep that in mind ;)