There is a significant difference in the two rounds. The WSM is an 80 grain case vs 56 for the 308. I think it is the best 7mm out there when you consider its performance is on par with the 7mm Weatherby mag and it can be chambered for a Shorter action.
The problem with the 7WSM is it was released last and is not catching on. This may be good for you, I understand these guns are sitting in inventory and are not moving, you may have a chance to buy it right. You can load these downand still be above 308 ballistics and the brass will hold up really well also. The WSM would be my choice due the fact you are reloading. The 308 does have the advantage of being offered in some extremely light rifles if that is critical.