I went with 450 Bushmaster.
450 Bushmaster Upper, DPMS lower
That said they are ballistically very similar. 458 SOCOM has a slight case volume advantage 61.6 gr H2O vs 60 gr H2O. Where 450BM runs slightly higher pressures 38,500 psi vs 35,000 psi.
As for bullet selection I think the edge goes to 450BM. Go to Midway, Graf and Sons or any other online reloading supplier. They all carry way more variety of .452 bullets than .458 bullets. Yes some of those are not really made for 450BM terminal velocities but many of them are and for plinking ammo 45 ACP bullets are cheap and work fine. The bullet makers are also making more and more bullets specifically for 450BM. In addition many bullets made for the magnum 45 cal pistol cartridges, especially 460 S&W are excellent choices for 450 BM bullets.
And if you're willing to do a little extra work resizing .458 diameter bullets down to .452 you have an ever larger variety to select from. Resizing can be accomplished with two $20 Lee sizing dies and a good closed arch single stage reloading press. Step down to .454 then a second sizing with a .452 or .451 die. I found they spring back a bit and after ruining through a .451 die they mic'rd at .4515-.452. It's allot harder and/or more expensive to go the other way. I used resized .458 405gr SP 45-70 bullets to work up a very nice subsonic load for my 450BM.
Left to Right: Barnes, 200gr XPB, Barnes 275gr TSX, Hornady 240gr XTP/MAG, Hornady 300gr XTP/MAG, Hornady, 225gr FTX, Hornady 250gr FTX, Remington 405gr SP (resized).
Now if I can just get that deer to hold still for a second.