Which one do you prefer? Why is that? We can see in gameplays that these skills have the same mechanic, but looks, sound diffferent and deals the other type of damage.
If anyone haven't seen Ray of Frost before - here's the video: http://www.youtube.com/watch?v=gTRK6t8ZXn4#t=1m12s
As far as I'm concerned, I think RoF looks better and I love frost skills more than fire ones (and here's the paradox: Disintegrate is supposed to deal arcane, not fire damage).
Also, RoF has a chance to freeze enemies, disintegrate not. BUT we still don't know if runes will introduce a huge twist connected with these skills! :)

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Q u o t e:
BUT we still don't know if runes will introduce a huge twist connected with these skills! :)

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They do.

The core skills are pretty similar, except yeah, ray of frost will slow/freeze enemies. But with runes they diverge in a lot of cool ways.