It really depends on what aspects of the game you find to be the most fun.

I'm personally going with a PvE team for instances and eventually heroics. They'll level from 70 to 80 mainly or entirely in instances.
My shaman team of five is 67; I'd like that to be 70 and run BG's and four of them do Arena as soon as possible. This team will be for pvp.
I have a hunter team, I think 53 now; I'd like them to hit 61 before my RAF runs out, so I can boost two toons to 58 levels each. The hunter team is for farming and dailies - not the most fun aspects of the game for me, but it will support what I do enjoy.