Precision AR upper -- caliber?

[Mwinter]

I have a bit of a dilemma....either way it'll work out fine but I'd like some input from folks with actual end-use experience with the following:

I've decided to allocate some funds for a precision AR upper. The idea is that I can put a precision upper (18+ hbar, scope, etc.) onto my patrol rifle lower (tricked Bushie with good trigger).

I have a 20" hbar .223 upper, but the .223 doesn't thrill me as a precision marksmans cartridge. Plus, since the price per round of top-notch .223 is currently about the same as other, better rounds, why not.

I'm thinking of flattop upper in either 6.5 Grendel or 6.8SPC, with a Trijicon Accupoint or other midpriced ('round $500) scope on it. This would allow me to use my patrol rifle as a more accurate and more capable precision rifle.

I could spend the dough on a Remmy 700 and scope, but why not get the most out of the rifle I'm already carrying and training with?

Info appreciated guys. The Grendel has my vote so far over the 6.8, but honestly inside 200yds who cares. I want info on specific companies, barrel lengths, rounds, etc.

Thanks!