Personally...and your mileage might vary, I'd stick to 5.56. It's a NATO cartridge, surplus is still available, and you can chamber both .223 Rem and 5.56 MM in it.
I'd get a traditional AR, the pistons are nice...but with proper lubrication and maintenance, a regular AR will run and run and run.
6.8 is a great cartridge, and it was pointed out to us that it is not a simple upper swap today in armorer's school. In reality, one needs a complete upper with new BCG, different magazines (although I hear 5.56 mags can be used, just not 100% reliable)...so it's not that easy. The lower is the same, just differences in other parts. Ammo is sort of hard to get unless you wanna shop on line or you have a forward thinking shop in your area.
The course I attended was S&W, and I own an M&P-15, they are tackdriving guns with a lot of great manufacturing features. They are predominately LMT innards, with Troy flipups on those models...so quality there. That said, I also have a CMMG with a DPMS lower that I put together, and it's a shooter too. I'd avoid Olympic for QC issues, Rock River for long wait times, Bushie for QC (in my opinion, too many guns too fast...but again, my opinion), and Colt because you are paying for the Pony...that might get me banned for being blasphemous. Stag makes a good product, DPMS, Armalite, LMT, LWRC, S&W, CMMG...you name 'em...they all shoot.
We have an ORC on the shelf, it's not anything you can't do with any other gun, and it does limit you by not having a front gas block with a Picatinny rail, it's just the block.
Good Luck to ya!