Easy to split Pirlo and Xavi. Don't get me wrong, Pirlo is a great player. Near unparalleled vision and long passing, very graceful style.
The thing is, Xavi in his prime was on a whole other level of dominance and influence. He was everywhere on the pitch and controlling everything, the heartbeat of two teams that made history. His ball retention, movement, and passing range is on an entirely different level from other legend CM's like Scholes or Pirlo. Absolute domination in the 08/09 treble, the driving force behind all of the NT's title wins, and probably the most significant player of the greatest footballing system in history. Assisting in 2 CL finals, 2 Euro finals, dominated the WC, holds numerous assist/passing records, etc.
Spain has two absolutely world class central midfield talents right now in Thiago and Koke who will probably go on to the top of world football and mark an era the way many iconic players have done before. A prime Xavi combined the engine and movement of Koke and the sublime passing and vision of Thiago into one player. That's how significant of a player Xavi was, that it'll take the output of two players to mimic what he alone did.
Pirlo on the other hand is very static in comparison. He was easily marked out and suffocated by Bayern through Kroos/Muller/Mandzukic in their CL tie in 12/13 and he ended up with something like a 70% pass completion and an utterly muted performance. Same thing for Italy by Spain in Euro 2012. He knows how to pull the strings and create plays from deep positions but he doesn't dominate a game, not the way Xavi does.