I have a large GoG library to complement my Steam one, and I do love having a nice clean .exe file with no strings attached, as well as the bonus material which is often top-notch; however I still prefer to have my games with time stats. Achievements are nice too, though I'm not too overly concerned with those. The biggest issue will be possible Steam Workshop support. It'll be hard to say no to a Steam version if that's the case, but as long as all the mods are offered on a site like nexus, I'll be buying it on GoG.
Nexus Mod Manager is compatible with Witcher 2