Better selection of bullets with the .458 Socom and headspaces on shoulder so trim to length and crimp are not as critical are the major differences. The headspacing on the mouth is what killed the deal for me on the .50 B. When I got into the .458S game, it and the .50 B were brand new on the market and I really wanted the .50. If nothing else, at that time 35 years of reloading everything from plinkers to elephant guns told me that I really wanted headspacing to take place on the shoulder so if need be I can crimp as much as necessary to keep bullets from either unseating or getting set back in the case. I went with the .458 S and have never looked back. As it turns out, a crimp is not really all that necessary unless shooting the 500 gr. bullets at supersonic velocities, then they tend to get beat up a little in the mag. But my reasoning still stands. If something happens and you trim a .50 B or .450 B too short, that case is trash. No problem if they headspace on the shoulder, just trim them all to the new length and adjust your dies and you can keep on shooting. This actually happened to me when I first started loading for the .458 S and the figures were kind of floating. I had gotten one set of figures and trimmed accordingly only to find out that those specs were a bit out of date and the new specs were about .005 longer in the TTL. If that had happened on the Beowulf or Bushmaster I would have had 100 unusable cases. Not so in the .458 S. In fact, I am on my 10th loading of that lot of brass and they are still going strong.
While this is no longer the case, at the time there were very few bullets in .451 (or .500 in my case) that could stand up to the higher velocities of the rifle round and that could be a real problem. Since the SOCOM uses real full diameter rifle bullets it was the only logical choice for me. If doing over today, I would still choose the SOCOM over either the Beowulf or the Bushmaster for pretty much the same reasons. Larger selection of bullets and headspaces on the shoulder. The SOCOM using standard mags was nice but was not a deal breaker to me then, but would be now. And I am not blowing smoke about choosing it again. I currently have 4 firearms chambered in the .458 SOCOM, a 20 inch, 16 inch, and 12.5 inch (SBR) rifles and a 14 inch bolt action handgun.