Re: which 300
Ballistically there isn't enough difference between the 300 WM & WSM to matter. The Win Mag has more factory options & the WSM allows you to save a couple ounces with a shorter action. If it's a mountain rifle, WSM makes sense, if you don't hand load there is an advantage with the WM. Which of those matters most to you?
Now if you open your comparison to the .300 Weatherby, .300 RUM & Weatherby .30-378 we could talk about major differences in ammunition availability, cost, performance, etc..