Looking at the blizzcon arena tournament (like the one yesterday) there are no warriors - and that is for a reason. You see alot more DKs for sure.
Warriors are currently in a bad position- perhaps the worst ever.
So unless you are aiming for gladiator titel / top rated arena , pick what you enjoy the most.
The strongest is imo DK.
However, balancing can (and will hopefully) happen, and perhaps one of the two gets buffed or nerfed, which changes things around.
Personally the strongest melee class (as I assume you want to be melee) is either rogue or ret paladin. Might worth consider if its just a alt for pvp