It's not about "playing deep" or "playing high". Positionally Xavi plays as high up the pitch as Iniesta if not higher up at times. Prime Xavi was more trequartista than regista. Del Bosque won 2 tournaments using Xavi as the most advanced central midfielder. Here is the average positioning in Barca's last CL final:
Xavi is as much "attacking midfielder" as Fabregas although Martino, like Frank used to do, misuses him closer to Busquets because his brain, like 90 % of all coaches, works in the traditional "4-2-3-1" scheme. The problem is not about where but rather
how they play. Fabregas is more erratic with his touches and passes, he doesn't read the game as well as our other La Masia midfielders, his movement is much more aggressive and archaic (rushing into the box to get into goalscoring positions every opportunity) and his defensive game is worse. He's more athletic and energetic and he might even cover more ground defensively than Xavi but Xavi reads the game and blocks the passing lanes correctly 9 out of 10 times whereas Cesc gets "stuck in". Overall Fabregas is more chaotic and hit and miss; not a great tactical brain or organizer either. If we want to play with Cesc and Iniesta, which is perfectly possible (although I made Cesc sound like a scrub so far but that wasn't my point
), we need a much stronger defensive unit. But that would be also true for Vidal.