I debated quite a bit on the big bore upper and always wanted to try one. I went with the 450B because the upper cost as well as the intended purpose of a 100-150yd powerhouse. All in all the three, 450B, 458S, and 50B all act about the exact same out to that distance anyway. When i totalled up the cost of getting the upper, 100 pieces of brass, 100 bullets, Dies and reloading accesories, and 2 boxes of loaded ammo, the 450B was cheaper by about 150-200 dollars and that made my choice for me. To me it will be like the Ford vs Chevy vs Dodge debate, everyone will like one and hate the others but they all get the job done.
Alot of folks hang up on round count in the mags as well but with a little modification on standard mags the 450 will work and hold just as many as the others, if you do a little research on the web you can find the how to's for that modification.