Neither. Short mags seem like a gimmick to me.
Don't get the "gimmick" factor?
A 270 offers a certain level of performance in a required long action rifle and for point of reference say with a 24" barrel. A 270 wsm doesn't require a long action making the gun shorter and faster cycling, from a terminal performance perspective the wsm will match the performance of the 270 but with a 22" barrel or it will exceed the performance of the 270 with a 24" (but because of a shorter action it is still a lighter faster rifle).
So, if you happen to live in a hunting area where short fast handling and fast cycling rifles may be considered an advantage, the wsm are just that, equal performance with about 3" less overall length and a slightly faster cycle. And every bit as competent and arguably better in long range performance considering most feel short fat cartridges burn more uniformly compared to long tall columns. One could also argue that shorter actions are stiffer.
If the 270wsm came out 80 years ago and the 270Win just 10 years ago, well the 270Win wouldn't even be brought to the market, what would be the point. It would offer nothing over the wsm. It would truly be a gimmick. Nothing wrong with using functional old technology but if I'm buying new at the exact same price I'm going to go for the better value. The 270 Win has become the gimmick, no functional advantage over the wsm.