Sprites look better, two extra boss fights, aditional art galeries and sound test, better music in one or two levels. MM8 PSX is still good, but the real version is the SS one
Also, some of the stages were retooled a bit (namely Tengu man's). It's a lot of little stuff that just kind of adds up. It's just a shame though because the additions and changes are cool but not hundreds of dollars cool. Also, with Saturn emulation just generally not being great really the only way to play the Saturn version is by plunking down a bunch of money for it.
Dream scenario for Megaman Legacy collection version of MM8:
-It has the typical legacy collection stuff like art galleries, sound test, challenge modes, etc.
-It better default features the better sprites from the Saturn version (with an option to turn on the PS1 versions explosion effects and whatnot)
-It features cleaned up FMV anime cutscenes (I guess the PS1 version would have to be where they pull the FMVs from as they definitely looked better on Playstation)
-Cleaned up audio (Again, they would have to pull mostly from the PS1 version as that version definitely has better audio)
-Finally, the game should have two difficulty options: Normal would be the PS1 version exactly as it was just with the aforementioned audio & visual improvements and the other would be Hard (or Saturn mode?) which would be the Saturn version exactly as it was but again just with the audio & visual improvements.
The above would be awesome and because of the people that would be presumably making it I could almost see something like the Saturn version of MM8 being on this collection.