Post by klaugh
I'm relatively new to mage so feel free to take what I post with a grain of salt. Personally I am PvP oriented so I'm almost entirely frost, level 39 atm, with eventually a few into arcane.
Leveling is definitely faster with the questing and level-cost changes introduced in patch 2.3. There are a lot of proponents to aoe grinding, but personally I find that boring and like the challenges of (or at least goals to attain provided by) questing.
Frost provides you with the ability to survive when other mage builds would likely crumble. If you get jumped by someone/thing in stealth, as a fire mage you haven't got much, your strongest ability has just been taken out of the question and, I regret I know little about the arcane-only build. You have frost nova, permafrost, frostbite, cone of cold, ice block and cold snap, all by level 30 which are highly useful. Sure, you miss out on the pure damage that fire brings to the table, but with +frost damage gear, you can get very nice damage and still have the high survivability rate. I've found I'm able to take on elites my level while alone with frost build, where I wasn't with fire build.
In PvP, in bg's the slow-down is great when there are other casters around because it gives more nukes before the mele get to the clothies, and the occasional frostbite stops someone and allows you to blink away into safety, not to mention the frost bite, frost nova, and chill effect are great flag stoppers in WSG since they have to pop a cool-down in order to escape them, the more there are the more likely one of them will stop them. Or if you want to carry the flag yourself, post-40, you get 'double the bubble' to absorb damage for you, even though you're a clothie, so you can have ice armor on, pop an ice barrier, when that dies, pop a mana shield, pop a second one if ice barrier's cooldown hasn't finished yet, then when ice barriers cooldown is done pop it again and repeat the process, a mana pot and decent mana pool and you can survive for a while running like that since both bubbles are instant cast.
The build I personally will likely end up with for pvp is shown here
With this there are still two unspent points to put whereever you want, I know I know, 2 points isn't much, but it's something. But it gives the few things in arcane that are rather useful in PvP; clearcasting, arcane impact, arcane fortitude and imp counterspell, while still providing a powerful frost tree.
With frost, definitely look for gear that is "of Frozen Wrath" at your level.
+Frost damage > bigger mana pool
Hope any of this helps.