At the later stages you'll want them to be the same for better synergy.

As stated above, Frost is the better for Solo survivability. For multiboxing though, I think you can get away with blowing it all away before it gets to you :)

I'll have to say though, I am sucker for Frost. Always played frost on my mage, and would die tons anytime I tried out Fire. Also, I really love the way the Frost Tree builds on itself. Either will work, frost has more survival, fire has more damage. I would go with two of the same though.