well if your not talking about a raiding scenerio... ie gearing up for heroics and maybe some 10 mans later down the line, there is a difference. if you are lvling your mage, i HIGHLY suggest you level deep frost. there are many reasons to do so (i can't wait till dual spec comes out and i can have my offspec be deep frost so i can aoe farm again).
as frost its all about the spellpower, and haste. crit is nice, because you have a decent crit multiplier, but you also get 10% more to your frostbold anyway. for frost dmg>haste>>crit. however, more stam and int help greatly as you level.
on my mage (1270 sp 140 haste 270 crit 160 hit @lvl 70) replaced very few pieces of gear lvling up. i however saw many items that were slightly lower dps wise, but had a lot more stam and int.
to answer your questions:
the items you pick up will have lower dps stats, but much higher base stats. that is going to be the case predominantly as your lvling up. your mage is in T4 lvl gear if i remember correclty. you probably won't replace anything (except trinkets) until lvl 76 or so.
at 80 i would stay as frost to do dungeons and heroics until you get all lvl 80 blues+ i would reccomending having a nice balance of crit and haste. haste will be worth more, but spell power is worth about 1.5x crit or haste. the elemental precision bug still exists (and works with frostfirebolt), so you get 6% hit from EP with your frost spells only (your fire spells only get 3% with the ep bug).
for leveling i was using mage armor, and with the improved mage armor glyph, that allows for 50% mana regen while casting, and this makes solo leveling go much faster, because you have significantly more uptime, and you get some extra resist to magic. in 5 mans, where you don't have a reliable mana regen source, mage armor is nice. glyph'd molten armor provides 5% more crit, but 50% mana regen while casting is very nice (i remember being arcane specced, and whith glyph'd mage armor i was at 80% mana regen while casting. too bad the damage rotation now doesn't use enough mana for that to be useful... arcane blast is kinda broken atm), but 50% mana regen is amazing for uptime.
There is more to life than pew pew you know, like staying out of the fire so you can live to pew pew another day