I use fire cause it generally does more damage, where frost just kills stamina lol. I do love Ice Spear cause it looks brutal, I wish It did more damage though. I think the whole destruction school is heavily underpowered when compared to Oblivion.
We're in Oblivion General
For me it depends on the character. I rarely use Destruction as it is, but I've had characters that specialise in one form of elemental damage or another. My personal favourite would be Frost though, due (I think) to starting the TES series with Morrowind and needing to get around the Dunmer's fire resistance.