I enjoy Feral more, but I'm concerned about how Ferals will do in Vashj'ir. Will the whole underwater-thingy cripple melee-leveling? I'd like to think not, because there are classes that don't have the option to be ranged, but I'm still a bit anxious.
But in the end, I think it comes down to one thing: whatever you enjoy the most. I know that I will level as Feral, even if it would suck to level in Vashj'ir, I'd just suck it up out of personal preference. And the "worst"-case-scenario would be leveling in Hyjal. I could live with that.