Got a manual? If not, you need one. Amanual is absolutely essential. In any case, you can load both with the HS6. If you insist on using two different powders, my old Lyman book gives 11.0(934fps) to 15.0(1280fps) for a 150 grain cast bullet out of a .357. The two grain won't matter.
There's data on Hodgdon's site for the HS6. No W's.
data.hodgdon.com/main_menu.asp
You will be better off using .357 brass for both loads. Eliminates the lube gunk build up in the cylinders. Mark the .357 loads with a felt pen. Seat the bullets flush with the case mouth for the .38 loads. No crimp. And to the bottom of the groove with a light crimp for the .357 loads. That'll give you another load difference indicator.
