I have two 300 Win Mags and 3 different 300 WSMs. I handload and shoot all of my loads over an Oehler 35P chronograph. Ballistically, there isn't a nickels worth of difference. Shooting 180 grain TSXs for example, the 5 guns all shoot within a 100 fps, but the fastest and slowest rifle use the same case headstamp. When I change powders or bullets, a differnt rifle has the fastest load, but again, all pretty close together.
The biggest differences are in the rifles, not in the balllistcs. My Kimber MT in 300 WSM is the lightest of the 5, and a great little mountain gun. It hits me harder than all the rest, but is a couple pounds lighter than my Win 70 Supergrade in 300 Win Mag, so this is to be expected. I love my supergrade, so still hunt with it a bit, but for remote backcountry hunts I tend to the lighter rifle. The rest fall in between, and I have a gorgeous 300 WSM that is really a work of art, but almost too nice to hunt with. I used the other 300 Win Mag (Kimber alos) on my elk this year, and am starting to really like it as well. I have chosen to select loads (150 grain TSX in the Kimber MT, 180 gr TSX in the Kimber 300 Win) that suits each rifle well, and suited to the game I am likely to pursue when carrying each rifle.
Handle the specific rifles in question, think about your hunting situation, and be happy with either cartidge. They really are very equivalent.
Bill