He's 3 and a half years younger than Santi though, I personally think Mata will go on and be the better player of the two when he reaches Cazorla's age.
I actually rate Santi, but he's not "clearly" the superior player. Mata made the Spanish bench while the other didn't, for example.
It is kinda unfair to judge Juan based on last season where he was the only player making things happen for us, a lethargic, slow team with almost no movement up front, god help me if I was a playmaker in that kind of team.
Yet he showed great flashes of brilliance throughout the season. I think him along with Ivanovic and Cech were our best performers last season.
Hazard, Oscar and co will only help his play IMO.