While I truly love the look and the platform of the PTR91 myself, I think the AR10 is just the more practical choice due to aftermarket support. It's also the original AR platform. The AR platform was originally designed around the 7.62/.308 NATO round, not the little 5.56. If I was to buy an AR rifle at all today it would be an AR10 not an AR15. For CQB I would drop right down to 5.7x27 or .30 M1 Carbine territory, in a subgun/SBR type of format.
But....that's just me.
With that said, my only "modern" battle rifle is an AK47 I built myself, with a Swedish Mauser doing sniper duty on the side. And I ain't too worried about what I need to have next, if ya know what I mean.