I'd say the element depends on the weapon you're using.
For glaive/spear I would go fire. You can't attack quick enough to have a constant frostbite on them.
For blade/sword it depends on what you're doing. Because you're attacking so much even if they pill they will be refrozen. However, in PvE this is useless, as your goal should be to kill as many as you can, as quickly as you can.
As for bow, from most bowmen i've talked to, they switched from ice to fire because later on the damage is lacking. The ice may seem tempting at first, but in the long run it's nothing but water =D
But as a rule of thumb, I believe bake said it best:
Quote:
Fire if you want to do more damage, Ice if you want to take less damage.