I think a couple of things need to be given to Pirlo even though I think possibly Xavi filled the role better.
Pirlo basically re-invented the role, he was a traditional, baggio esque no. 10 struggling to live up to the hype when he was bought by milan and moved to the back of the diamond. This transformed him and also brought back the deep lying playmaker role that I can't think many other teams utilised. Scholes was used far higher up the pitch than Pirlo, though he dropped back sometimes.
So while I think Xavi dominates games a bit more, Pirlo does need to be given some credit for re-invigorating the role he plays and providing it's modern blueprint. The other thing is that Pirlo went very stale at milan, the midfield went unchanged too long except for the loss of Kaka, which is the last thing you'd want to do to a midfield, so he had quite a few years where he wasn't at his best.