Personally, I think Darius Plus is not only easier to find, but pretty much the same game as Super Darius (sans the extra 2 bosses). But the game itself is hard on both, so once you have one, you'll probably end up ignoring the other one anyway. I have both, but only coz I found Super Darius at a flea market, tucked away with some music CDs and was too amused by the fact that a PCE CD-ROM game would find it's way there.
Visuals: Virtually the same (kind of a credit to Darius Plus being a very impressive Hucard port of the arcade; like how R-Type was), with very minuscule differences with Super Darius. 1 or 2 extra bosses in Super Darius (CD).
Audio: The music on Super Darius (CD) will obviously be in much better Stereo, provided your AV out works fine. The tracks themselves are taken from the arcade version, which uses different instruments than the 16-bit chiptunes from Darius Plus. But honestly, I actually prefer the Darius Plus (Hucard) music, since it's louder, crisper & less muffled & muted like the arcade tracks. Question preference, I guess.
Other criteria: CDs don't last as long or keep as well as Hucards, so you're more likely to have your heart broken by Super Darius when it conks out, than by Darius Plus which will last virtually forever since the contacts on the cards can always be cleaned and wiped; CDs are much more delicate.
Hope this helped!