IWB has a double hit, both hits can crit, and at the same level with the same gear IWB does more damage that IB.
As P/m you have other skills to weave in so the 3 second cooldown isn't a problem, fireball, bone chill + lightning, then ice wind normally take care of most mobs at level.
P/m in my opinion is a very capable priest for soloing if you're statted for it, I used to run P/m instead of M/p for questing because it was more survivable.
If you are P/M/S, try both and see what you like using best.