Wow, just toss a coin! To be honest pick a gun first and then pick the caliber. I really love my Browning 300 WSM. With a 180 grain premium it'll do all I need from it. However I don't need to shoot 200+ grain bullets from it, I have my 375 H&H for anything big and bad that I can't take with the 300. Yeah, the older 300 mag does work better with those heavy bullets, but we would be just "picking the fly$#!+ out of the pepper" to worry about it. Pick whichever you like better and don't look back, in the real world there isn't enough difference between the two rounds to worry about. However, there is a difference in the rifles they come in so that would be my main concern.

Gordon