It's short action, instead of long- For me, the difference is not having to lift my head up off the stock while reloading, and keeping cheek weld and my eye on the target. It's hard for the eye to re-aquire and re-focus every shot, if your scope is dialed in on magnification...especially for hunting follow-up shots at distance.
Also, the performance specs on the .300 Win Mag and the .300 WSM are very similar, with a half-inch difference in case-length, so the rounds are a bit lighter as well. I got mine as a trek/hunting gun. However, i think that the .300 win mag ammo is easier to come by - which these days is something to think about.
However, that being said, i was hoping Armalite would come out with a left-handed AR-30; like their LH AR-50...i would have gotten one. That didn't happen, though. Then it would have been a toss up between the .300 Win Mag and the .338 Lapua. I think the Lapua is 4 bucks a round minimum, but it can reach so far!