You can headspace the 300WM off the shoulder after the brass has been fired once. Also, I don't believe the WSM will run with the regular WM. Factory loads for the WSM tend to run on the warm side and with my reloaded 180gr. Accubonds in my WM, I get 3150 with no pressure. I did a ton of research on these two rounds before I had my 300WM built and found out that the 300 WSM fits pretty much right in the middle of the 30-06 and the 300WM. The benefits of the short action are over-rated too I think. You only save around 4oz. in weight and a well built long action is just as accurate as a short action. Don't get me wrong, I'm a huge WSM fan. I own a 270 WSM and have seriously considered having a 300 WSM and a 7WSM built. After a years worth of research, I decided on the 300WM and I'm waiting on my 7RM to be built.
Now all that being said, I don't think any animal will notice the difference between the WSM and WM. Shoot which ever one turns your crank.