AdamTaylor: At this point I might just end up buying the GOTY on Steam when it's on super sale (as they often do). I'm very disappointed in GoG right now.
Then you'll be disappointed in Steam also, as the Steam GOTY version is a mere bundle. I would link it, but I'm not sure if that's allowed.
Steam's policy / Steamworks are a hassle regarding fully-integrated & original versions. It's the reason why Dishonored and many others have simply bundled GOTY versions on Steam.
As for the topic.. I have the original game + season pass, and I prefer it that way, as I want to control which expansions I install, or rather when I install them. But I do admit that I kinda want to have the integrated GOTY version "just because", haha.