Fred gives good advice.
I would have to say though, the only area that the 300 Win Mag shines above the 300 WSM is when using 200 grn bullets. The 300 WSM actually produces more velocity with 180 grn bullets than does the 300 Win Mag. It's a more efficient cartridge. The bullet seating depth has nothing to do with long or short actions. It is all about the style of your magazine or magazine well and how long the barrel throat is. This does not matter much because the standard seating depths are already producing superior velocities with both the 165 and 180 grn bullets with the benefit of better accuracy.
Anyway, if you are looking for an accurate 300 magnum, you can't do much beter than the 300 WSM.
You should base your cartridge on what quarry you plan on pursuing.