If you are using RAF, it doesnt really matter. the first few quests will ding you to 60 and you will be right on track with the leveling curve.

That said, the longer you can hold out in azeroth, the better. Just finish up feralas, winterspring, and the plaguelands, and head to outland at 60+

I leveled 2 teams, one went through the portal the moment I dinged 58, the other at 60 and it didnt make much of a difference to be honest. Non RAF, if you go to outland early, you have to either do some gathering quests, or run ramps to make up a bit of exp. If you dont have the ability to do ramps (less than 5 chars, no 70, etc) u might have to grind regular mobs, or end up hitting zangarmarsh a bit early.