You can't really say with any real definitive answer.
Different way of playing, different formation (in the main), a different make-up in midfield.
Selling him would also have enabled us to make other signings, just like selling Raggett presumably allowed us to sign Bostwick and Anderson and selling Margetts allowed us to sign Whitehouse. There's always a chain in there somewhere.
Currently top of the league would suggest we aren't missing, but he'd strengthen the team, no doubt about that. We don't currently have anyone in the side who does his job, but again, we're playing differently, so is that kind of player as necessary as the previous two? And as SincilBank states, everyone would snap your hand off if we were about to sign a, currently, top L1 sides Captain - just like we did in January with Frecklington.
As Gaz has already mentioned, Posh fans rate him incredibly highly already. Many are saying he's the one enabling the others to do their thing. Perhaps if we had Andrade/Akinde and pre-season fit Freck and Pett it would have played out differently last season.
In summary, there's no right or wrong answer to the questions because too many things have changed!