I can't chose because for my uses they do a different job. I think of WW231 for real light loads in handguns. When I go to Unique I'm usually going for heavier loads. I love 231 for the 45 Colt because I can use a bit less than 7 grains and attain the thrifty 1,000 shots per pound of powder goal I like.
I have an 8lb can of WW231 I've been shooting out of for years and it seems to have no bottom. My Unique supply shrinks much faster for some reason.