I have just done this, I went with the 7mm mag for the following reasons. I didn't want a muzzle break as this will be a bush stalking rifle as well. So 7 mm mag won because of less recoil.
In NZ getting brass for 7mm mag is easier than 7 stw.
7 mm mag easily fits into a long action with projectile seated right out. 7 stw might not fit if you want the projectiles right on/in the lands, as with the VLD projectiles.
My hunting partner already had a 7mm mag.
I am really happy with how it worked out. Here is a link to what I did Re-barrel to 7mm rem mag