Witcher 3 is an amazing game, but it is very different from destiny. It's hard to draw comparison between the two, making it near impossible to point out what makes one of them better than the other.
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The Witcher 3 is a massive single player game, like Skyrim and a great one at that. It's a genre that I am really into and can consume me, they just envelope me in its world and lore. Destiny is a always online FPS game with limited social aspects. I would in no way call it a MMO as it doesn't fill that criteria. The main draw is its spot on FPS mechanics and rewards, not it's story (story?) or its world.
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I would argue that the open world rpg medium is the better medium at portraying a great story. There are dozens of great story based rpgs while I can think of only a couple great story driven fps games. The pure gameplay is what keeps me coming back to destiny, and that is what matters most in the genre.