I've been meaning to pickup the MSX games, but the multiple versions confuse me...
Here is the 'splanation.
Hydlide 1 MSX1: monochrome sprites and stuff, on a cartridge. Password based w/ volatile save ram (If you turn the system off, your save state dies). Its still extremely legit, and I prefer it over the NES on.
Hydlide 1 MSX2: Same game, but on a floppy. 256 color mode (looks better than NES), and lets you save to disk. Get this one if you can. It usually costs more.
Hydlide 2 MSX1: It saves properly, and its fun. I skipped class to play it last semester, lol.
Hydlide 3 MSX1: Ugly.
Hydlide 3 MSX2: Not Ugly, and the same exact game.
Its kind of like MSX1/MSX2 version of Drasle Family (legacy of the wizard). Same exact game, except the MSX1 one has monosprites.
Hydlide in general doesn't get the credit it deserves