Well, IMHO nowadays they're equally good if you ask me. When they were here, I considered Silva being a bit better than Mata, because Silva made more last passes and his position on the field was more relevant, scoring more goals than Mata and being more popular than him 'cause Silva was Villa's right hand. That made Mata being not so notorious, but when both Villa and Silva left Valencia, Mata was more predominant, more important for our game, totally replacing Silva's game and skills, and being more mature, more clever on his movements, having a better eye for the passes, etc.
To put it simple, when the three of them were here, the specific position of each one was this:
------ X ------
X -- X - X -- X
X - X - X - Mata
----- Silva ----
----- Villa ----
Now, as I said, they're both excellent players, but I prefer Mata over Silva for various reasons:
- Mata can play as a left winger or as a link between midfielders and strikers. Silva can do it too, but tends to do only the second thing. Silva hardly ever played as left winger; on the other hand, Mata has played as that intermediate player from time to time so he has that kind of experience. In a few words, I see Mata being more capable or doing different jobs on the field, thing that Silva can't do because he centered his game on being the last passer and nothing more.
- Mata is more professional in terms that he wants to play always, Silva only played here when he wanted to. When Silva didn't want to run, that was like we had only 10 players on the field. Mata, instead of that, "sweats the T-shirt" like we say in Spain, runs all the match and gives the best of him always. Silva is more like a self-called star, and Mata is kind of humble, if you catch my drift.
- Mata knows that there are more players from his team on the field, Silva lacks that. Silva prefers to rely on his dribbling skills and plays "solo" most of the time, and Mata, however, prefers to pass the ball. Probably Silva is, then, faster and capable of dribble the defenders, but I think Mata's style is more clever and more convenient for the team.
To put it all together, Silva is a superb player being the man who does the last pass, but nothing more. Mata maybe lacks Silva's keen eye for that job, but I see him more regular, more all-around kind of player, capable of doing different things depending of which position has on the field, and personally I prefer a player who is a B on everything than another one who is an A on one thing but a C on the others.