There are no bugs I know of in either game. Thunder Force IV for Japanese MegaDrive is a little bit harder in places, particularly the Gargoyle Diver boss--his globe moves VERY fast. This was altered and slowed down a great deal in Lightening (sic--sorry, it's just dumb and should not have rightfully happened so I continue to write this, I'm still rather pissed about the renaming) Force, and the alteration stuck for the European Thunder Force IV release.
Thunder Spirits is indeed a Super Nintendo version of Thunder Force AC, but interestingly one of the stages is different, presumably the system could not handle it so you get treated to a different/additional spacecraft scene for a stage. The final boss is also quite a bit different, once again it is likely that technical matters dictated the difference although there is the possibility someone just wanted to try their hand at different artwork. There is a lot less animation in those scenes though.
