The M99 Barrett, and AR-50 are both great choices. I went through over a year of searching and shooting every 50 Bmg I could before buying mine. I bought a Bushmaster BA50. The reason being I love the left hand bolt idea and shot a carbine model. It had very little recoil. It has the least recoil of any 50 bmg I have shot. I needed this being I have back issues and recoil causes more pain for me than most. The downfall is the weight, it weighs 34lbs. with a full 10 round magazine. The reason I picked the BA50 is I wanted a match chamber and barrel, lower recoil and magazine fed. I wanted a bolt gun and it wa between the Sig Sauer 50, McMillian Tac50 and the Bushmaster BA50. After shooting the Tac50 and Bushmaster side by side I picked the 30" barrel version of the Bushmaster. I have never thought twice about my decision. I was hell bent on buying a Barrett 107A1 until I found out that there are way better shooting 50's out there. They are ment to shoot fast. After a vist to the Outpost Armory and talking with numerous Barret salespeople, the semi auto Barretts are 1.5-3 MOA. I actually had an M95 bought and decided to let my best friend buy in due to higher recoil. But the AR-50,M99,M95,Tac50,Sig50,and Bushmaster are way better shooters than the semi Barrets. This coming from experience and from Barrett sales persons themselves.
Optics.... NightForce is a great option. If you can spring the money USO,Premier, or Schmidt & Bender would be my top picks... If not NightForce,Leupold Mk4 or Bushnell Tactical Elite are the next best. All are built tough enough to handle the 50's recoil(not so much force but the muzzle brake the breaks other scopes). When the gasses pass by the muzzle brake it forces the rifle forward and that is usually what tears the scopes up... I have no experience with the benchrest models.
If I can help answer any questions feel free to shoot me a message... I shoot 50bmgs on a monthly basis at least.