I'd take the rifle bullet option every time over the pistol bullet option.
There are some very good hunting bullets in .451" so this advise is not iron-clad and without debate. But overall the SOCOM is a better option in my opinion.

Everything you can do with a pistol bullet from the 450 Bushmaster you can do as well or better with a 300 GR H.P. from the SOCOM, and there are things the 350, 400, 450 and 500 grain .458"s can do the Bushmaster can't.

Yes the SOCOM uses standard 223 mags.
I am unsure about the 450 Bushmaster