I'd go with the 7mm wsm if I were hand loading, if I was buying factory ammo it really doesn't matter, since more ammo is being produced for the 7mm wsm now, but I am an expert by no means. I don't understand the concept that the short fat cartridges are a fad. A short fat column of powder burns more efficient period. I have had a 7mm mag for the past 12 years, last year I bought my wife a 300 wsm, almost the exact same case as the 7mm wsm. The price was right or I would have bought the 7mm wsm so that we could shoot the same leads. In chronographing velocities with the same powder, the velocities of the 7mm mag varied from 10-60 fps, and the velocities of the 300 wsm varied from 5-25 fps. This happened consistently over 50 rounds of hand loads for each. If that matters to you I would go with the wsm (short fat case), if not find something that matters to you weight, etc. and go with which ever satisfies.
I would in no way sell a 7mm mag to buy a 7mm wsm. They are so close that it doesn't matter. The 7mm wsm will usually be a little lighter, but the lighter you go the harder it kicks. To say that a 7mm wsm kicks less maybe, but you would have to have a machine to be able to tell the difference, everything being equal.
Don't worry about which to buy considering the case. Look at weight, length, and most of all how it feels when you through it up.