I myself am on the fence. While a majority of my games exist on Steam, all of my IE games and D&D games are found on GoG, as well as the Witcher series. It'll also be where I probably install TW3 from, since it comes with more goodies and a better price than Steam can offer. It will also be MY copy of the game, which is kind of nice to know, and not just a licensed copy.
So either you have Steam:
- More active forum base
- Easily take and upload screenshots
- Your own copy, not licensed
- Doesn't require internet access or downloading patches and setting to offline mode
- You can download all of your goodies in separate packages
- Collection with similar titles
Of course, if you're like me, you can access your preferred copy of the game on your preferred platform, and unlock your second copy (hero edition) on the opposite for those benefits, if you don't have anyone in mind who would play it.