I have a 450 B and love it.. I bought it years ago when ammo was pretty much non-existant for 458 Socom and 50 Beowulf..
the biggest decision is ammo choices.. the 450b pretty much only has a couple hornady/remington factory rounds available in the 250 gr weight, but reasonably priced around $26/box
the 50 B ammo used to only be made by the gun mfg and it was closer to $40/box when you could find it.. and it has more bullet weight choices..
you can buy the bolt and barrels to build you won 450, but would need to open up the ejection port a little..
My 450B is a great Virginia deer gun - sighted in correctly its +/- 2" out to about 200 yds.. with the proper hold over you can go 250 to 300 yds..
good luck
b