It's also a question of context - Mata stood out for Chelsea but the Chelsea side of those two seasons was probably the worse of the Abramovich era (and, yes, I know they won the CL, somehow). They failed to mount any kind of title challenge. Likewise, he was playing for a team of relative also rans in Spain. So we still don't know if Mata can be a key component of a team with ambitions to win the title. I'd say, based on both club and international record, Di Maria is the more proven performer at elite level.
Definitely agree with this.
He scored lots and assisted lots, but it was poor Chelsea Team:
They didn't have a proper system, they were gung-ho, they were ridiculously open due to him (and others) being given no defensive responsibility. They didn't play a possession game or dominate teams, or show any control.
He shone in a team that played from back to front quickly (conceding lots of goals, and losing too many games in the process), and where he had space to run into.
He also didn't really dictate their play, and even if one believes that he
did - it was hardly great play.
For a supposed 'no. 10' I've yet to see Mata actually dictate or run anything worthwhile in his years here, and also have doubts that you can build a successful team around Mata, or even make him a key part.