They are actually very close and it turns down to preference, price and once again, role you want to assign him.
Sandro has a big jumping and heading advantage over marchisio, but other then that, they are fairly evenly matched. The thing that has more going to Sandro would be him being 3 years younger with very similar potential and stats.
but yes, i'd generally choose sandro over marchisio. I prefer the defensive approach when it comes to my Defensive midfielder. I play rigid philosophy generally and therefore if i want a defensive midfielder to do the defensive role, i will take my odds with John Obi Mikel and Sandro. If i wanted a DLP i'd go for Poli. And even then to get the best of a DLP you generally want a ball winner, hence getting sandro sitting as DM beside him.