Marchisio is a rare player, Pastore will most likely become an elite player, a Ballon D'Or material, something March would never become. But if Juve got relegated or failed to finish in the top 10 for few years, you know that Pastore would pack his bag and leave for the first offer, while March wouldn't.
Marchisio is from the same breath of the likes of Giggs, Del Piero, Zanetti, Maldini and Totti, definitely not in terms in talent, but in the way he's committed to the club. I would say he's Juve's version of Gary Neville or Carragher, not the most talented but very loyal and loves the club.
It's harder to find a player who truely loves the club than to find a player who's world class.