All in all I think I'd have to go with Xavi, I think he probably edges in terms of goal threat and the ability to find space. Although Pirlo is the better free kick taker.
I think both have been recognised in their latter years as with Scholes as the DLP role has kind of garnered more attention and the plaudits look behind the AM's to see who keeps things ticking over, and even by managers, ****ing Roy Hogson is just cottoning on the fact Carrick might actually be of use. But Xavi was performing at that level earlier than Pirlo imo, perhaps that is because of the team he played at and Barca being a bit ahead of the curve with the balance of the midfield three.
My vison of Xavi is clouded anyway because I'm a big Iniesta fan and I think he gets sidelined by the Xavi hype sometimes.
Xavi also has the second most simple yes massively trick I've ever seem, just behind Valencia's "Am I coming inside? Nope byline!!!" That little 360 he does where he just turns round and then keeps going, I just cannot figure why it works so damn well.