This is a rather "old" thread, but IŽll go ahead and answer this because this question pops up kind of often when people stand at the arena vendor scratching their heads.
Alright, basicly, MH+OH will offer you slightly more SP. However, staff will offer you more regen stats (Spirit). In ideal world you would want to have both MH+OH and staff, to switch for whenever you are turtling, but in general MH+OH is better mainly because of melee speed (Yes, melee speed), to wack them totems on the run, hiting that orc caping the node, but mainly for totems. The use of it has been decreased however, since the change to tremor totem, but you still want to wack down grounding totems etc in arena against shamans.
So in short, go for MH+OH first if you are into PvP.
And yes, spirit is good for a disc priest, however how you stack it, depends on the comp you play.