I did call PTR and was told that they resolved the issue of non-GI models not being able to run tar-seal surplus ammo.
But looking at their website under "technical information"
G.I. Series Rifles
Made with match grade barrels
Chambered for .308 or 7.62 NATO caliber
Rate of twist: 1 in 10
15mm x 1mm right handed barrel threading for attachments
Original G3 “stepped” barrel profile
91 Series Rifles
Made with match grade bull barrels
Chambered for .308 or 7.62 NATO
Rate of Twist: 1 in 10
15mm x 1mm right handed barrel threading for attachments
Barrel diameter: .70”
Fluted barrel
So while they may have changed some specs on the 91 series rifles to be more reliably feed ammo, it seems only the GI series have the "original G3 stepped barrel profile. Don't know if that is a better option or not?
I don't mind not having the rails on the hand guard. But having a rail welded on the receiver for optics is really nice!!
Lastly I like the painted finish better than the parkerized.