For a bit of frame of reference, when you're doing quest mobs such as Gorefiend's armor or the Forge Camp:Hate/Fear boss elites, the extra damage mitigation you gain from frost barrier will pale beside the extra DPS output of fire/arcane. Frost is awesome DPS for soloers that can freeze the target and make use of the Shatter talent. Fire/arcane just dumps the DPS on 'em regardless. Also, fire/arcane DPS will be MUCH more likely to keep aggro on the mages when you start having to spam heals on them than a frost build would.