IMO, the 300 win mag is a better caliber than the 300 WSM. The WSM is harder to load for
* Most of your reloading stuff doesn't work well: loading blocks, powder funnels, chamfer tools
* Some have chambering problems
* They take less shells in the mag depending upon the model
* You can't neck size cause they develop a crush fit after the first firing
* You can not load them for as much velocity, especially with the heavier bullets because the bullets start intruding upon case capacity
* Components and equipment are not as available as the win mag
* The develop pressure problems in a hurry and never, ever load above book max in a WSM
On the positive side - case head separation is not as much a problem without the belt and the excessive headspace on new cases.
I have reloaded for one of these and they have a 26" barrel and shoot pretty good. There is enough room in the mag to load against the lands and if you decide later to change it you have a nice Remington 700 action to rebarrel (like maybe to a 300RUM or a 6.5 rem mag).
Just load a 300 win mag up with 76 gr RL25 and the 200 gr Accubond for about 2950 fps and you will have a real winner. The 300 WSM will be more problematic.
Your results might vary.