With Invocation as it is (and possible even after the proposed changes mentioned by GC hit the PTR) all 3 specs feel bloody horrible. I find im running out of mana well before the invocation buff has expired, somewhat to be expected with Arcane.
RoP is MUCH better as far a smooth rotation is concerned however the regular argument on movement comes into play here. Also with the amount of evocating i find my self doing and my recount numbers on the target dummies. RoP is pulling well ahead of Invocation for all 3 specs.
Using RoP on a target dummy i have recount showing much higher numbers for Fire followed by arcane then frost a very close 2nd. I could probably have squeezed more out of frost im just a bit rusty with ye ol frost.
Looking at the gear available on the PTR im seeing crit gear available for every slot outside of tier. The only slot of tier that doesnt have crit is helm and legs.
At this stage im confident that fire will be the better spec to play in 5.2