Bigger open world, which is what OP asked for. And more polished combat.
They're both fantastic games, but weirdly, some people prefer City to Asylum. Be careful about "fixing" people's opinions.
Aha just a bit of fun dude
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I actually think Arkham Knight is the best of the 4. Certainly has the best narrative imo and the combat feels that little bit more refined. Though I think Asylum is a more well rounded experience overall.