If it's between Xavi and Modric:
Longevity: no comparison. Xavi was done by 34 like most modern all-time great CM's. Modric will turn 38 this year and will probably extend with Madrid.
Athleticism: a mismatch. You put both on a running track and Modric would completely dominate Xavi on all distances from 100m to 10.000m. He would beat him in jump events too. This is reflected by their style of play and yes, it does matter, just ask Enzo Fernandez. Imagine Xavi and Pirlo in the same situation. Peak Modric keeps up with Adeyemi and no goal is scored.
Trophies: close call on club football. Xavi has won more domestic titles, Modric more UCLs. International trophies, obviously Xavi, but a WC final and 3rd place finish (and thus making history) with Croatia is almost as impressive as Spain's run between 2008 and 2012.
Individual awards: comparable although Modric's Ballon d'Or and World Cup best player award should give him the edge.
Aesthetics: this is very subjective but Modric by far for me. Way faster than Xavi, lighter on his feet, run past the opposition with ease, master of the trivela passes. Modric is just beautful to watch, he's a workhorse and an artist at the same time. Xavi is more basic and a little bit robotic. I'd rather watch Pirlo or Iniesta.
As far who is the better actual central midfielder when it comes to all the things a player is supposed to do in this position, this is once again very subjective. But I'm going for the more multi-dimensional player and that is Modrid. If I'm a club owner and I can buy any midfielder but I have no coach and no players yet. I'm taking Modric because he can play in any system that exists whereas Xavi's lack of athleticism could be exposed in systems where you don't have the possession.