I know that Sissoko would improve more in the near future. But, there is a point here, we don't have a pure destroyer in our squad bar Zanetti and Giannicheda who will leave in the summer, also he isn't up to Juve's level. Marchisio can make a good defensive midfielder, but Sissoko is already a solid one whom we can depend on. At the end, we'll have more available choices for the midfield and this is vital for a long season. We'll most probably play in Serie A, Coppa Italia and the CL after one season from now. Therefore, I think that capturing Sissoko's signature should be done as soon as we can.
Also, I rate Gilberto Silva as one of the best defensive midfielders in Europe right now, and a realistic target too. But if we get Sissoko and we failed to get him, this won't bother me. As I believe in Zanetti's capabilities, he has the required experience and the fighting spirit. Yet, his only problem is being injury prone.
As for Emerson, I think I'm leading the people who won't welcome him back.