Update: It is currently not confirmed if Steam keys of Witcher 3 will be redeemable on GOG.com... Witcher 1 + 2 are redeemable, but until officially confirmed, assume that Witcher 3 is not.
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Is there any difference in terms of content and future DLC avb ?
The FREE DLC program for The Witcher 3: Wild Hunt consists of 16 entirely free DLCs, that will be available for every gamer owning a copy of Wild Hunt, for Xbox One, the all-in-one games and entertainment system from Microsoft, PC, and PlayStation®4, and is applicable to all versions (digital or physical, standard or Collector’s).
There will also be two paid expansion packs for the game in the future. The first expansion, Hearts of Stone, is slated for release in October. The add-on sends Geralt into the wilds of No Man's Land and the city of Oxenfurt to complete a contract from the Man of Glass. The second expansion, Blood and Wine, is scheduled for release during the first three months of 2016.
The Witcher 3's paid DLC expansions will be available as part of the game's Expansion Pass, which will retail for $24.99. If players want to purchase the add-ons separately, Hearts of Stone will cost $9.99, while Blood and Wine will cost $19.99.
The only extra
digital goodies you will get are exclusive to GOG version of the game:
8 wallpapers + world map + 6 paper toys + exclusive tracks (MP3) + exclusive tracks (FLAC) + 5 HD videos + 16 avatars + 24 artworks + comics (Reason of State - Part 1) + paper toy catalog.
Note that if you buy the retail Standard Edition of the game you get the following
physical goodies, in addition to the game discs and the GOG-exclusive digital goodies:
An exclusive CD with the official soundtrack
The official, developer-created "Witcher Universe - The Compendium"
A detailed map of the in-game world
A set of unique stickers
A protective sleeve