mpthole got it right...
Serbus are the best value for the buck I think.
If I ever actually buy a .50 it'd be a Serbu. They are light,(as far as a .50 goes anyways) and they are bolt action, which is much easier and quicker than a shell holder design.
Havent shot an Ultramag, but it looks cool.
And a thing about scopes on .50's...
You can put any scope you want on a .50, but if you dont get a high quality scope (Leupold,Nightforce,Swavorski) its just a matter of time till the rifle EATS it. Im currently on number 3 .
It even ate a Leupold 4x14 tatical, but at least Leupold gave me a new one. Right now Ive got a Tasco World Class 8x40 that has about 400 rounds on it. SO far...so good. I dont consider the Tasco to be in the same leagues as the others, but at least it hasnt broke yet.