The ballistics of the 6.8 put it around the class of the 270 for a lot more cost. That plus the cost of Barrett stuff make it a pretty pricey toy. I think if you like 270 ballistics, which are pretty darn good, then get a 270 in a really great bolt action rifle and call it a day! Jack O'Connor would love you for it!
If you want something that out performs the 5.56 NATO, we already have THAT, and it is the 308. I purchased a DPMS LR-308 over a year ago and love it, with the following caveat. You must be willing to buy or reload 168 grain cartridges for it. It was absolutely lousy with 150 grain bullets. It is a tack driver with the 168 pills. I can also say that I have been impressed with the customer service at DPMS.
My vote would be for a 308 rifle. It's a more "serious" round. The only question at that point would be, do you get an AR-10 platform or go with a M1A? Either way the 308 is hard to beat.
I have heard stories that many of the combat squads in the Middle East are requesting that some old M14s be unpacked and handed out because the 5.56 NATO is not up to punching through crap to get at targets. Not a problem for the 308, or a 30-06 for that matter. Hmm, how do you feel about a BAR?

That's my 2 cents.
EZ
The rifle itself has no moral stature. It may be used by evil men for evil ways, but there are more good men than evil, and while the latter cannot be shown the path of righteousness by propaganda, they can certainly be corrected by good men with rifles.