Imo the difference between Macherano and Sissoko is in their heads. The former seems to be the smarter one and that makes him the better DM at the moment.
Mascherano is moving very smartly, without needlessly wasting his energy. He is always in the right position mostly because he's a smart fellow that has the ability to predict where the ball will be passed or what will the opponent try to do. In this aspect he reminds me of Maldini of some 10 years ago who also intercepted tens of passes during a match without running everywhere like a headless chicken.
Sissoko on the other hand seems to waste his energy, and he does it a lot. He runs where the ball is and he's prepared to run 90 metres just to make it harder for the left back to pass the ball to the centre back.
Luckily for us Sissoko has a lot of energy and it's incredibile how well physically prepared he is. This year he spent most of the first part of the season on the bench and he's not tired now, but with the way he's not saving himself at all, i'm affraid that we'll see him suffer in the second part of the season next year.
If only Sissoko used his energy more smartly.
But then again....maybe he won't be the player he is if he does that ?!!
P.S: Btw, one friend of mine, a Liverpool fan, was talking to me about some Liverpool match where Rafa started with Mascherano-Momo at the center and for 30 minutes the other team didn't pass the midfield, not even once. After that all they did was playing long balls for 60 minutes, because there was no other way to pass through Momo-Masche.
Anyone remembers this match ?