I think that in the low to mid level bracket it really makes no difference. Fire or Frost. I have been slowly leveling a Frost Mage, and I kill almost at the same speed as a fire mage, but my mana usage is why better. This means that while a fire mage kills mobs a 3 sec cast before me, I make it up in being able to kill X number of mobs more before resting or popping evo.
As for instances, WoW is so pathetic with low to mid level instances now, I often will 3 man them with the right group, which is Tank, Healer, and any dps. (Sometimes with two tanks even.)
I remember the good ole days when you were doing your first VC or WC run and you were all nervous about group dynamics and how do you do this or that and the number of wipes. Now it is run, as fast as you can, get blue drops, rinse and repeat once or twice.
Last in the low to mid level instances if you like being top dps as a frost mage, you just have to know when and what to cast as it is not so simple as casting primarily fireballs and fireblasts with a scorch or two. Tends to be a bit more usage of all 3 magic types to be on top of dps, but only if your group is stopping to mana up occassionally.