Being a 6.5 nut, and, truthfully, not being so fond of the AR-series (blasphemous, I know
). I'd recommend the AR-15's. AR-10's are a more exclusive club, as already posted, and their ammo WILL be more expensive. AR-15's have a MUCH better aftermarket "upper" selection, the base ammo is cheap, and getting a cheap "plinker" upper is easy business (either .223 or .22LR).
If you want to go hunting, get an AlexArms 6.5 Grendel upper - MANY in the 6.5 community have yet to really have a good argument against it, as it is a 6.5x55 built for a semiauto AR-15 platform. For the price of an AR-10 AND an AR-15, you could (much cheaper) have a single AR-15 with a 6.5G upper, and be all set for any purpose. Or, as stated - the 6.8SPC (better for sub-300yds, AFAIK though). Also, the Grendel has been doing well in 300yd+ shooting competitions...
For "Grendel Takedown Power", follow link:
http://www.youtube.com/watch?v=CwtOBLrmLiw
Plus look for other videos. It's truly a remarkable cartridge, this coming from a die-hard 6.5 Swede fan, and self-proclaimed "Euro-[british cigarette]"
For the AR-10, there is the Creedmoor, but that's been criticized as a barrel-burner as of recent...which is never a good plan.
Oh, and to really get your jollies-rockin', there's the .50 Beowulf uppers for the 5.56/AR-15 platforms. Again, versatility...