No, it's different. I claim that Melo offers more to the whole team. As players, I think it's rather silly to even compare them, because Marchisio clearly isn't a defensive midfielder.
What type of player is Marchisio? He isn't a winger, clearly. He isn't a regista too. He's a central midfielder that has limited vision, passing abilities and has very poor off the ball movement. He can be compared with Inler IMO, and if it comes to that Inler is better at almost everything and not just over the past year.
I wasn't the one who compared them. I agree with the most of your post. However it's not always easy to compare players who are in teams of differing success, if that point wasn't obvious think Marchisio at Juve pre-December and then afterwards. And no he wasn't always average this season, but his personal decline with our team also puts questions against his character. I think to what Jack said earlier, how he used to rate Marchisio up until the last two seasons. Two of the worst Juve seasons ever. Probably not a coincidence. Hell, Buffon and Chiellini can get criticised to shit on an off game in this Juve, so I take it with a pinch of salt. Marchisio isn't a fighter character like Chiellini, so maybe he deserves it.
For large parts he was tactically sacrificed, unlike Inler who was steady and settled all season. When I saw him otherwise withdrawn in the centre three or two it does make me wonder what his instructions are. It doesn't compare to previous performances in terms of tempo, which makes me question what Delneri expected him to do. I agree he is a tactical anomaly. Personally I see him as an off-trequartista, he is better getting at players and linking off one-twos then getting into the box. His natural abilities suit this. He's not a destroyer, not a dictating playmaker, not a winger, closer to a box to box player. But our tactics last season didn't suit that, it was two wide men with a DM and a playmaker.
I've seen Conte's Siena and if his Juve is similar there will be a place for Marchisio, but it'll have to be the driving Marchisio. And yes, it will be one of team sacrifice again, more workmanlike that flashy. There shouldn't be players who have a definite starting shirt for Juventus anyway, you get in on merit. One of our problems last year were that players were often automatic choices through default, and this breeds complacency.