I have several friends who have asked me this same questions ... 300 wsm vs win mag.
Just like everybody else has said, it is probably hard to notice the difference in recoil between the two in equal weight rifles.
If you never plan to reload, and the rifle you are looking at has a 24 inch barrel, then I would go with the WSM. To reap the benefits of the extra case capacity of the win mag, you need a 26 inch barrel and hand loads. Otherwise the muzzle velocity between the wsm and the win mag are going to be the same with factory ammo. And it does appear that wsm factory ammo is pretty easy to find these days. For a factory wsm, Barnes Vor-TX shooting the 165 TTSX is tough to beat. Most all of my friends hunt with 300 wsm's and this Barnes factory ammo, and I've seen a whole lot of 1-shot kills on elk.