you'll find more info about the Eagle on the AR10 forum. Eagle is a division of Armalite so it can legally wear the AR10 logo. 5Subslr is the guy to talk to over there--that dude knows more about the AR10 than anyone I've ever met.
Personally, I like 'em. They're cheaper than the Armalites, have a 3 yr warranty (I think) instead of the lifetime that Armalite offers, but they're essentially the same. All the aftermarket parts designed for the Armalites fit the Eagle as well.
I don't own a 50 or 458, so I can't compare--I don't reload and 1000 rds of good surplus ammo only runs $170 or so....and you won't find anything on the North American continent you can't kill with a .308...especially with 20 rds