Aside from no DRM and additional content such us wallpapers, has the GOG version been updated to be more compatible with Windows Vista and newer hardware, or they just both play fine?

I am tempted by the Steam deal this weekend too. They should both play fine, Steam, much like GOG, uses DOSBox for old games, and Oddworld isn't too old to begin with.

I don't believe GOG does any actual coding. They always seem to use Dosbox just like Steam. My CD version of Abe works on Vista 64, without Dbox.

But does the Steam version working without the Steam client?

If the Steam version uses DOSBox, then it may not need the Steam client. IIRC, some 3rd-party Steam games do not require the client to run.

Steam version could be downloaded in English, Francais, Espanol, Deutsch, Italian.

When the Oddbox is released on Steam the games will feature Steam Achievements

Little update.

Steam supports only:
Windows 2000/XP

GoG supports:
Windows 2000/XP, Vista, Windows 7

That was why I bought here at GoG.

(It doesn't mean that the version from Steam won't work on Windows 7, but at GoG is confirmed.)