Deerstalker, I also have a .458 SOCOM. I made that choice due to having both a .45-70 and a .44 Magnum. I use the bullets I have for the .45-70, and the powder/primer that I have for the .44 Mag. The only thing I had to buy was the cases and dies.
I didn't even have to buy a shellholder, it uses the same one as my .308/.243/.30-06/.45 acp.
Logistically it was an easy choice for me.
The only powder I've tried in it is H110, but I want to try some Lil 'gun (which I use in my .44 mag rifle), and possibly some IMR4198 with some heavies.
To the OP, the .458 does not require special mags or mods to the lower. I think the only real mods required are opening the ejection port and the boltface obviously has to be opened. It functions best with a carbine-length gas system.
I don't know that much about the other cartridges, but I can't imagine there are any *real* field performance differences. I suppose that bullet choice would be the most important.
I do know the .458 does have some "whomp" factor to it.
