Right now I think Int would be best but I can see arguments for everything. Stronger shields are always nice, and we can get more mastery than Int from gems. I can also see haste though, due to the fact that we can afford to cast more spells.
I'm way too behind on theorycraft to make an educated decision, and I feel like if I could get someone to back this up with math it might get me going again. Any help here would be much appreciated :)