To be honest, warlocks are not very strong in arenas without an effective healer (preferably a disc priest would be nice). I have never had success in arenas without a healer as a partner.
However, to answer your question, they both have benefits. Destruction pushes out more damage, but demonology provides more mobility with metamorphosis. I would suggest demonology for the mobility. That being said, be cautious and use LoS to your advantage. In WSG, locks are effective CC'ers with their fears and dots (to keep stealthies in combat). However, in arenas it can be frustrating.
Good luck!!