You also have to consider whether or not you are going to "drive" your mage (druid, etc) solo. For example for mages, frost is the best multibox spec, since it gives 10% extra caster damage and its very easy to control/macro. HOWEVER, frostfire bolt spec (0/53/18) is vastly superior to frost for raiding DPS, especially once you hit 1600 spell dmg and 30% crit levels of gear. So if you plan to spend a bit of time raiding in guild or in pugs, you may just want to spec 53/18 since your DPS will be quite a bit higher when grouped with others. On the other hand, arcane is a vastly superior spec for arena, so if you are going to arena with friends then arcane is way better.

Just figure out always first what the thing you want to do most is - arenas, battlegrounds, wintergrasp, farming gold, heroics, regular instances, raiding, etc. "All of them" isn't really a valid answer, you are going to gravitate to one more than the other. I do all of them, but my team is setup more for heroic farming and doing raids with my mage/pally more than anything else, so I have a very geared tank (prot pally), a very geared AOE healer (resto shaman), a mage (teleporting/food), and 2 ele shamans (may replace one with a boomkin) - remember, the challenge isn't going to be the current 5 mans, but the NEXT 5 man that opens up in a few months.