I would go with the .300 Win Mag. There is a larger selection of factory ammo for the .300 Win Mag (68) versus the .300 WSM (34).
Those numbers come from Midway USA website.
Those numbers are perhaps a bit misleading, as they include types that are unavailable. For "Available" the numbers are 47 and 18. If you includes "Coming soon" and "Out of stock, Backorder OK", the numbers are 56 and 19.
Not criticizing, as I often refer to the total numbers as well, in the belief that they are still a relatively valid indicator. The .308 @ 123 and .30-06 @ 99 lead the pack.
In terms of cost, the cheapest WSM ammo is listed at $31.99. Midway lists 11 types of WM ammo from $23.99 to $31.99.
