First off they are
absolutely not interchangeable.
The difference between the two is that the 7mm Rem Mag is a long action belted case. The 7mm short mag is a short action bottle necked case.
As factory loaded there is little velocity difference between the two at all.
Here are the WSM's and their long action cousins. As you can see they are totally different case designs and will not interchange with each other.
Just to all a little more confusion for ya. There is also a short action ultra magnum by remington. It is similar to the WSM's but is not interchangeable with either of the other two above.