Hi, honkey~
Good question. Here's how they compare.
Both cost virtually exactly the same for a 1-lb bottle and are made by the same company (Alliant).
Powders are broken into basically three burn rate categories: Fast, Medium and Slow.
Fast powders like
Bullseye are used for very light target loads thru medium velocity for a given cartridge. Contrary to what you might think, "fast powder" DOES NOT mean it will push a bullet out the barrel the fastest. On the contrary, you will never be able to safely load
Bullseye and a bullet to the maximum velocity the cartridge is capable of. To try to do so will cause overpressures which can cause tremendous damage to the gun and the shooter.
Bullseye measures for the 158-gr LSWC bullet range from "very light" 650-fps loads with about 2.8-grains of powder, to "medium velocity" 750-fps with about 3.5 grains of powder. So while it isn't capable of faster loads it is very economical in that a pound (7000 grains) of
Bullseye can load maybe 2,000+ cartridges.
Bullseye is also inherently dangerous (particularly to beginning reloaders) because the very small charge weights allow a double or even triple accidental overcharge to fit inside the case and if not caught can be catastrophic. A big plus is that
Bullseye burns very cleanly at all loads and leaves very little residue behind.
Here's what
Bullseye looks like (this pic is actually of Alliant 2400 but Alliant
Bullseye looks almost exactly the same). Being somewhat finer-grain than Unique it measures a bit better and very consistently with very little weight deviation from round to round.
Unique is a "medium rate" powder and is made up of larger flakes making it bulkier and more difficult to overcharge. Being a medium rate powder, it is best used for achieving mid-to-heavy loads as it can 1) get bullets up to higher velocities and 2) burns completely only at higher velocities. If you tried to load some 650-fps target loads with
Unique you can certainly do it, but without the pressure of recommended loads it doesn't burn completely and leaves a LOT of soot and dirty residue in and on your firearm.
Unique is a truly outstanding powder and is still economical although for .38-Special 750-fps loads with your bullet you're looking at 4.5-grains and for 850-fps +P loads you're looking at 5.0~5.2-grains so a pound of powder doesn't last quite as long as Bullseye. (Either powder costs only a few cents per round so I never sweat the cost)
Unique is
excellent in both 9mm and .38-special handloads. It is coarser so it fills up a case more and makes triple charges impossible and double charges very unlikely.
So in short,
Bullseye for maximum economy plus clean burning, but the trade-off is slower velocities and a risk of accidental overcharges.
Unique for faster velocities
only in order to burn clean and less chance of overcharge, but you can't load it light and it's coarseness makes it tend to measure with +/- 0.2-grains accuracy which is still totally acceptable and it is not quite as economical if you are figuring your "per-round" cost down to the tenth of a cent.
Oh, and SLOW powders are generally used ONLY for very high velocity loads like the magnums. Slow gets a bullet going the fastest but has to have a LOT of pressure to burn completely which means the bullet is going to really zing.
I use very little Bullseye, and a LOT of Unique for my 9mm, .38-Special, .45Auto and 20-ga shotshell.
Wow, I really didn't intend for this to get so long winded............ But it's all information needed to answer your question.
