The .300 WSM is a good round, and does tend to be more a little more accurate than the .300 Winchester Magnum. When the .300 WSM came out, Nosler bought a test barrel to do their handload pressure-testing on their indoor range. Before that they used the .300 Winchester Magnum to test their heavier .30-caliber bullets for accuracy, and the .30-06 or .308 Winchester to test the lighter bullets--but switched to the .300 WSM for testing all their .30 bullets.
However, some of the publicity stuff about the .300 WSM was BS. Winchester claimed it could match .300 Winchester Magnum velocities with less powder, due to the "efficiency" of the WSM case design. At the time the velocity of the standard factory 180-grain load for the .300 Winchester Magnum was 2960 fps--and the standard .300 WSM matched that. But the .300 Winchester load operated at lower pressure. Eventually .300 Winchester 180 factories appeared that got well over 3000 fps, more than the .300 WSM can do.
The .300 WSM is a good cartridge. I've owned three rifles chambered for it, and hunted big game from pronghorns to elk with it, but it does kick noticeably more than the .30-06, and since .300 WSM rifles tend to be a little lighter than .300 Winchester Magnums, their recoil is similar in factory rifles.
Since the original excitement over the short, fat beltless magnums (SFBMs) has died down considerably, it's harder to find .300 WSM brass and ammo--though both tends to be more available than any of the other SFBMs that appeared around the same time--some of which aren't available at all anymore. Meanwhile, .300 Winchester Magnum brass and ammo remains pretty available, even during shortages, partly because it's a standard world-wide cartridge.
