From a pure numbers min/max standpoint, probably warlock. But as the poster above said, unless you are in a strict progression guild (who would have already informed you of what you'll be playing, I bet) go with fun. I have both a lock and hunter at 90, and though the lock can pull better numbers at times, I find the hunter more "fun". I can't even define why - the lock has flashy spells, great dps, 3 totally different specs, yet I find myself coming back to a trusty ol' hunter time and time again.
I have both hunter and warlock at level 90. However I find that the warlock class receives so much more polish and is overall more polished than any class in game. Warlocks also do some of the greatest DPS that is available and offer this in 3 distinct specializations so you are bound to find one you'll love. Besides that they bring more then enough utility to stay competitive and have some cool gimmicks like portals/gateways.
They also have an awesome class quest for green fire (an awesome experience you shouldn't miss out on while it's still current).
Then we have hunters. I don't know, I loved leveling my hunter but at 90 I just can't get myself to play him. It's kind of the same with rogue, was fun leveling but once I am level 90 I lost all interest in it. There's just not much to being a hunter, no flashy stuff, no gimmicks, not as well polished, the specializations aren't different enough etc.
In the end, just go with whatever picks your interest more. You both have them on 90 so swapping between them shouldn't be an issue. In case you want to do progression, play with both now and see which one you end up more so you don't regret your choice in the end.