The case capacity of the WSM and SAUM are not the same. The WSM holds about 9 grains more water than the SAUM and will give slightly higher velocities at the same pressures. The SAUM might be fatter, but the difference is so small as to make it meaningless. So far the WSM has proven to be a much bigger seller and ammo is much easier to find. I'd bet on the WSM staying around and the SAUM disappearing. If you're a handloader then ammo availability isn't a concern so buy what you want.