There's not a whole lot of difference in the two. If you've got a keg
of Herco for shotshell loading and need to load up some .45s, it'll do
that. Same with Red Dot or any similar shotgun powder. They'll do,
in the absence of other, more specialized, powders. As far as
metering goes, flake powders tend to have more variation from those
fixed cavity measures like Lee's than ball powders. There's a reason
a lot of the early measures had a knocker attached. For cartridges
like .45 ACP, you probably won't notice the variation unless you're
set up with a full-scale test lab outfit. If you're NOT doing
shotshell reloading and are just polling folks to find out what to
buy, get a pistol powder.
Unique works for a wide variety of loads, probably more applications
than most any other powder. Not necessarily the highest velocity or
most accurate, but it'll work. If you want something more caliber
specific, you'll have to say what you want it for. If you were
restricted, for legal or supply reasons, to one powder type, Unique
would probably do.
Stan