I'm not overly impressed withe the 2 Fullfield IIs I had...and wouldn't pay that kind of money for one.
I don't know which DPMS .308 you have...I've got the LR model. I had the barrel cut to 20" and fluted...it really helped to alleviate the front heaviness of the rifle. I have a 10x Super Sniper on that rifle, which I think works really well.
I don't have any experiences with the Monarch line, but I love my Buckmaster...great scope for what I paid for it (dealer pricing at Midway--$237).
Honestly, I'd look at the Weaver Grand Slams...the glass is very nice on those, and they make a 6x20x40 version. I have one, and recommend this scope to everyone who's looking for that much magnification, and doesn't have the $$ for a Zeiss or higher end Leupy.
I guess most folks would consider a 6x20x scope more of a "varmint" scope, but there's no reason you can't slap one on a .308.
No warranty issues so far on any of these scopes that I own, so I can't give you any assist on that, sorry.