Does anyone know what the differences between the US & EU versions are, if any? Also, does anyone know which version GOG sells?

I'd imagine the differences to be localisation related. I think text strings can get a bit funky if you patch with the wrong version due to differences in expected character sets, but that's only mt best guess. I'd also imagine GOG to use PAL territory ISOs since they tend to be Multi-5 or Multi-7 which helps out in the wider market, but again that's just what I expect, it could be american preference or even regional dependent.