http://www.accurateshooter.com/cartridge-guides/7mm/
I like the info and opinions in the link above for the 7WSM & Saum. I have a 7Saum and I've used it combined with Berger 180s on several big game animals, including Elk and Eland. The WSM has a slight performance edge, the difference bertween the two is minimal. I can tell you one only big game animal took more than a single shot and that was my fault.
I think between those two calibers, the 300WSM and the 7WSM, the more important decisions are in the details. How you set up your freebore, what action, what system you use to feed the rounds, who builds it etc. I would pick the Caliber, pick the bullet you want to use and design the rifle around that bullet. Just my two cents.