I love big-bore ARs
. I'd probably go with the .458 Socom just because of the availability of bullets suited to these velocities (most all of them are designed for the .45-70). The .450 Bushmaster is even less common and is limited to largely pistol bullets. Plus the .458 is designed to be used with unmodified AR mags.
Don't know much at all about the .50 Beo, except that mag capacity is even lower (I think 5 rounds instead of 7, in a "20rd" mag), and bullet availability is very limited.