There is a HUGE selection of over-the-counter ammunition for the .243 Winchester. That, to someone that doesn't "roll his own" is a big advantage over the WSSM. At this point, Winchester is only offering three loads for the .243 WSSM.
The WSSM on the AR-15 platform will give you a little better performance than the .243 Winchester. The AR-15 will also be lighter and less expensive.
