Basically, fire or lightning?
(Ice for tanking, I know, but this is for mostly solo play)
Necro has a lot of AOEs, so the obvious answer is "lightning" because of the 8% aoe bonus, BUT... the aoes are dots (except for blastbones), and while they're ticking away in the background you're either spamming skulls or scythe. And since skulls is cheaper, that means the more sustainable choice is a single target attack with a high-powered ricochet ability every third go. So... would inferno staff be better for the single-target boost? Or would that single target boost be ultimately irrelevant?