I would lvl with frost, if you plan on doing a lot of pvp at 70 then stay frost with either a arcane frost mix or full frost, if you are going to mainly pve fire does a lot more dmg and is in my opinion more fun for raiding. I have the Battle Caster gear and it was the biggest waist of gold that I have done. I would get the spellstrike and fire gear and use your arena points for the rest. As far as fire build go I have done both Pom Pyro and full fire and I enjoyed full fire alot more for both arenas and raiding. I hope this helps

Kanna