If you are going to reload your own ammo,the 458 SOCOM is the way to go.You'll have a bigger/better selection of bullets to choose from. There's not much factory ammo available,SBR makes ammo but it is awful pricey.
The 450 Bushmaster and 50 Beowulf shoot pistol bullets,whereas the 458 SOCOM shoots rifle bullets like a 45/70 or 458 WinMag.
Reloading is the only way to shoot any of these calibers economical.
There are several of us here,that shoot the 458 SOCOM.It's a great round to shoot,and very fun also.
As far as brands-Bushmaster makes the 450,Alexander Arms makes the 50,and Rock River makes the 458.The 458 uses standard 223 style magazines,a 20rd will hold 7 458rds and a 30rd will hold 10 of them.
I don't know if the others use standard AR mags.