Also bear in mind levelling in PvP as an alternate source of experience.

I level primarily on BG weekends or CtAs and so it is quite viable for the 'time : experience gain' ratio. I find doing 5 of one class much easier in PvP

I will agree with Ualaa though, both types of groups are viable for instances, dependant on your classes you choose. My favourite for levelling so far has been 1 prot, 4 ele shams with a resto spec for each