When I first learned of GOG and I was still primarily gaming on Windows I tended to choose GOG over Steam since I prefer their DRM-free games. However since it became much easier to game on Linux (thanks to Proton and the work of Valve), I tended to start buying more on Steam since it was so much easier to get games working through their platform. Since Heroic launcher came out I have now switched back to primarily buying from GOG again.