204 AR has pointed out that it is possible to make a relatively light .243 AR-10. That got me thinking about comparing apples to apples.
You can choose to use a short, or light or even a short and light barrel. You can also choose to use a carbon-fiber float-tube, but there are some items on both platforms that you are stuck with:
The upper receiver: The AR-10 bare upper receiver weighs 90% more than the WSSM upper receiver.
The bolt/carrier: The AR-10 bolt/carrier weighs 69% more than the WSSM bolt/carrier.
The barrel-extension; The AR-10 barrel-extension weighs almost 3 times more than the WSSM barrel-extension.
Bottom line: Built the same, the AR-10 is going to weigh considerably more than the WSSM.
The AR-10 is also more expensive to produce. I have now dropped the AR-10 from my line-up, but from my last price for an AR-10 upper, compared to my current pricing on WSSM upper, the AR-10 was 20% more expensive to produce.
All that said, there are some real up-sides to the AR-10 and the .243 Winchester. Factory and after-market ammunition is much more plentiful and with more variety by far in the .243 Winchester. With the WSSM, you are limited to what Winchester puts out, and a few after-market suppliers. One of the after-market suppliers of .243 WSSM loads offers 9 different loads using premium bullets and at an affordable price.
The .243 Winchester is also much easier to load for. The combination of the thinner brass and smaller diameter makes it much easier to re-size.
The huge selling point for me is that the AR-15 platform is already so wildly popular. Folks that got their first "black gun" just because the wanted one, can push out the two pins and in twenty seconds be big-game hunting with a WSSM upper. And in another twenty seconds be back to plinking with cheap surplus ammo.