Vidal is far more superior in both role he plays, but especially when it comes to defending. You can see him being aggressive and dominating while that's not even the case from Marchisio.
Vidal has certainly gone up a level or two this season, while Marchisio is much the same player he was last season.
I doubt anyone would dipute that Vidal is the better ball-winner - but, then again, there is barely a better ball-winner in the world than Vidal just now - so Vidal being great doesn't mean Marchisio isn't good.
Vidal is also the player in the midfield really capable of stepping up and pressing in order to raise the tempo of the whole team.
However, to maintain the balance of a midfield, I would probably only have one Vidal in there. I don't want all my midfielder haring around and flying into tackles, because you are likely to be left exposed with two such players both stepping forward to press and being taken out of the game now and again.
I think there is very little between the two of them with regards to attacking. Both players are very productive when given the freedom to regularly get forward. I think Vidal is perhaps slightly overrated in this regard in comparison to Marchisio because of Vidal taking penalties.
Anyway, the bottom line is that Juve's midfield is excellent.
I look forward to seeing what they can do with a higher calibre of forward playing in front of them next season, which should enable them to look even better.