Maybe 50-100fps loss in velocity. Magnums, generally, and I'm assuming this holds true for the short mags (WSM's) as well, tend to lose a little more when cutting barrel lengths.
My own choice would be the standard old 270Win. which is good for mule deer out to 400yds., and is available in either rifle.
The 270WSM simply adds a couple hundred fps in velocity to the standard 270Win. Of course, the ammo is more expensive, little more recoil, more muzzle blast, etc. Personally, before going to the WSM route I'd go for a rifle in 7mmRem.Mag., but then you did say 270, and lightweight.