The .450 Bushy has slightly better terminal ballistics than the .458 does.
That seems contrary to other information I've come across. .458 SOCOM is supposed to be approximately equivalent to .45-70 loads for a lever gun. .450 Bushmaster seems to be about the same as a .45 Win Mag (plus about 10%). Coupled with the incredibly wide variety of bullets available in .458" (.458 SOCOM) versus those available in .451" (.450 Bushmaster) I would have to say the tip would go to the .458 SOCOM.