Let's get one thing straight, we're talking .50's here! None of them are LIGHT! The lightest .50 made is the one from Anzio at 13 lbs. unloaded w/o a scope and bipod.
If you think your going to be trotting through the woods with a .50 to go hunting, think again. Most hunters complain about their 8 lb. .308 rifles being to heavy. The Serbu carbine is 18 lbs. w/o a scope and bipod, so figure 22-23 lbs w/everything on it. About the only thing your going to want to do if you take it hunting is to find a good spot overlooking a wide open space and then sit and wait. This would be after driving to that spot with a truck or ATV.
Both rifles are capable of MOA or better with match ammo, Mark did a trigger upgrade on the BFG's and you can swap out the factory trigger on the AR-50 for anything that will fit on a Rem 700. With all this being said, most people will only shoot off a bench or prone with a bipod.
The reason I like my AR-50 over the BFG is the fact that it's just easier to shoot and there's less felt recoil. Basically it will come down to which one YOU like more.