PVP - Frost can control better. More survivable - especially later in the build. Fire is burst damage. Absolute best "Ambush" class/spec in the game. Massive damage... No real survival abilities (Don't even try to aruge that those useless 'stuns' will save you... haha) Not suicide, but if you get ambushed, you're likely going to be running back to your corpse.
PVP gets good in the 30s.
Farming - go Frost post 40. Get your mount, round up mobs, Blizz, Blizz, Blink, Blizz.