The Obliterate is especially good in conjunction with Hungering Cold.
If you're running with a Paladin healer, Concentration Aura + Aura Mastery will ensure you get it off.
It is single target, and won't break the crowd control with an AoE.

As to whether you should go with a 2-Hander or dual-wield, is up to you.
Dual Wield seems to be higher sustained damage, but burst is likely more important in Arena.

A 2-Hander will be larger Obliterate strikes and Necrotic Strikes.