Personally if I was buying a fast 6.5, it would be 264 WM, for no reason other than that cartridge has always intrigued me, way before the development of the 2 named in this thread. Just a "scratch the itch" issue I seem to have.
Of the 2 named in the OP, If I had to choose between those two, I'd go all out and get the 6.5-300