Have you considered the .300 Winchester Short Magnum, WSM?
I am thinking about this myself. .300WM, .300WSM, .338RUM
From what I've read the ballistics for the .338RUM are far superior to the 300's (other than 30-378 Weatherby and some wildcats). Sacrifice barrel life, and higher recoil, but lots more knockdown. Don't get me wrong, the 300WM is a great cartridge and definitely has some advantages over the 338RUM. As far as the short mags, other than faster bolt cycle rate with the short action, what is their advantage? When you are only talking about 3+1 capacity, hardly think cycle time is relevant. But, I'm definitely no expert. Thanks for the input ... I'll keep researching.