How do these cartridges measure up to each other?  Drop comparisons, etc.

I can't compare the ballistics, although I can say that the 300 has a slight velocity edge. The .30-338 has the accuracy edge (assuming, of course, that all else is equal).

From a logistics point of view, I think I go with the .300. Otherwise I think it would be nicer to have the more unusual .30-338. Of course, the new .300 short magnums offer .300 performance and in theory accuracy at least equal to the .30-338.

Edited to add: the short magnums offer less recoil, too.

For the best of all worlds, sskindustries.com offers the best of both worlds.

AR-10 in .300 WSM.