For one: The Steam version isn't available anymore ;)
I don't have the fakku version but i can tell what the illusion patch adds to the steam version:
- Steam version has lesser prebuild characters, basically all young looking dfc chars are gone (but you can still create lolis yourself using the editor or downloading chars from the internet) and the charecters have different outfit sets because:
- Some outfits are missing in the steam release, aka anything school related, school uniforms, schoolbags etc.
- The storymode is missing because it completly school themed
- Steam version has a different main menu background then the jp release
- I'm not shure on this but the jp release might be missing the mode where you can directly select a char and have sex und the mode where you can select any character giving an idol show
tl:dr; Valve hates Lolis
However there is a patch available directly from illusion which adds back everything. (Like this, you could say the steam version has some exclusive alternative prebuilt outfit sets ^^)
don't know if the fakku release is with the patch pre applied or not but with it also being named Party! i guess it's closer to the steam release than to the jp release.
but from what i have heared the koikatsu releases are way more intercompatible than the two honey select releases (FAKKU! pls fix).