Utimate? Hm...dunno. I haven't seen any of Havok's abilities so far in Ultimate.
Is he in the recent comics? I only buy the volumes, and the next one comes out in November. So if he's been in them recently, I haven't seen him.
Iceman, on the other hand, has shown himself to be quite powerful and versatile. He basically flash-froze a falling Sentinel before it killed many civilians.
Until I see some proof on how powerful Havok is in the Ultimate Universe, I'm gonna have to give this to Iceman.