for the 308 i like varget and rl-15.
for the 223, i like h-322.
all 3 of those powders will work in either gun, but you would be giving something up... in 223, while varget and rl-15 work, you will run out of case capacity long before you reach max pressure, velocity, or best accuracy (at least i did in all experiments i've run w/ various 223's and those 2 powders).
h-322 will work in the 308, and is actually fairly accurate, but velocity is really bad.
if i just had to have one powder for both chamberings, think i'd go h-322.