The AR platform might be lighter, depending on configuration, and will be more ergonomic.
I have a PTR-91 and it's a heavy beast. Yes, it works every time I pull the trigger; yes, mags are cheap; yes, it's hard on brass; yes, it's accurate with irons; yes, it's very front heavy; yes, it's harder/not as ideal to mount optics on; and no, I don't want to pack it around any distance. BUT it's potentially the cheapest, quality .308 "battle rifle" you can get ($900-1100).
I also have an AR-10, it is also a heavy beast but that's because of the configuration I wanted and built. Yes, it recoils less than a PTR-91; no, it's not as ammo sensitive function wise (heavy match loads here); yes, it's ergonomic; yes, it can shave a hair on a fly's ass at 100yds; yes, it's very optics friendly; and yes, it was very expensive.
That's my experience with the two. I'd highly recommend the AR platform over the 91 for your only .308. That said I wouldn't recommend the RRA LAR-8 because like it's father, the Bushmaster BAR-10, it's a compromise to use FAL mags; which no longer is a plus, the AWB is dead. DPMS is really looking good and face it, SR-25/orginal AR-10 pattern mags are the future of .308 ARs. DPMS also has configurations from budget, to "battle rifle," to precission rigs. The current Bushmasters and Remingtons are rebadged DPMS with added features like chrome lined barrels for Bushy and Mossy Oak camo for Remington. Armalites are nicer, IMO, rifles but they are stuck with their proprietary mags; but at least their current mag pattern was fitted to the rifle instead of the other way around.
ETA: servicing: you can build an .308 AR pattern rifle in about 10 minutes (assuming proper barrel nut wrench and action block) and buy every part individually, stock or aftermarket, for it and customize it to what ever you want. They are not standardized like AR-15s but the parts are available and reasonably priced.
You can buy HK parts for a PTR but you should beware of 922R. I find them more expensive and more difficult to work on.
I can't commit on customer service because I haven't had to use any of them.