Once again, the one we really want is the Sega CD version.
Appropriate time to link the recent HG101 Earthworm Jim article...
http://hg101.classicgaming.gamespy.com/earthwormjim/earthwormjim.htm
It finally answered my questions about what the exact feature set of the DOS version of EWJ1+2 was! Evidently it's pretty much the Genesis version with nicer graphics and the SE CD audio soundtrack and minus the level the SNES version doesn't have... that is, it doesn't have either the SE's added level or the level missing in the SNES version, it has none of the other added features of the Special Edition (saving, one new weapon, etc)... except for the soundtrack. Huh. But at least it does come with the only PC version of EWJ2...
But anyway, EWJ1 for Genesis is great, but it's even better with saving... and a CD audio soundtrack, and that homing rocket gun... but still, the Genesis version is absolutely worth getting, for people who don't have a version already and don't want to go out and find either Earthworm Jim SE for the Sega CD or Activision's Earthworm Jim for Windows 95.