Well, I ran 124 .357 magnum and 50 .38 SPL cases through the resizing die, tonight. Still don't have any .40 dies, and my Glock is still without barrel anyway, so I'll be loading just .357/.38 for awhile. Which is probably just as well, since revolver rounds are a little easier than autopistol rounds.
Will be shopping for bullets and powder tomorrow, so I better decide by then. 800-X would look better if IMR's published .357 load data went over 36,000 CUP (SAAMI max pressure is 35,000 PSI / 45,000 CUP).
It seems like Power Pistol and Longshot are just about identical in .357 magnum loads. Longshot is a bit more powerful at pressures under 30,000 PSI, while Power Pistol is more powerful at "primer falling out" pressures. Either should be relatively forgiving in .40 S&W. Seems like Longshot may have a slight edge, but it's still not a clear-cut decision. I guess I'll just get whichever is cheaper.
Thanks for all the input, everyone.