The 6500 also has more stable eARC connection.I came from a 6200 to the 6500 and there was a significant difference between these two units the biggest being AL32 over AL24 which has made a difference. Between the 6400 and the 6500 I think it's what's been dropped rather than added. No FM radio and some legacy connections. The inclusion of Alexa may make a difference to some, not something I would want or worry about.
Audio wise I really can't say, it's unlikely to be as great as the difference between the 6200.