First, yes it transfers over.
Graphics are the same on both consoles.
And not to sure about backwards compatibilty so far, I need more information.
But you can family share on xbox one which if your wife woukd play, you could create her account. She would therefore get all digital downloads like games and dlc you buy this saving buying a another game and Xbox subscription.
But if you want exclusives for destiny then ps4. That would be my only reason. But good luck.
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It shares across console on ps4 as well only one account needs ps+ good to hear that Xbox gets the same treatment.