They are both great players.
Both worthy of being the top 10 wingers in the Premier League.
But who do you think is better and why.

Different type of players. Valencia is powerful fast and doesn't stop running whereas Mata is very skillful and creative with the ball. I prefer Mata but that's my opinion. 

Mata is more creative, plays better passes, is more important for chelsea's game

valencia for his experience !!

@naman93: i think mata is more experienced, he played two euros, one world cup and more UCL seasons than valencia

if that's experience ...... then Pep Guardiola is more experienced than Arsene Wenger and Alex Ferguson !!

meh both have something to do with valencia atleast

@knibis hahahaha. No offense to the OP but I don't think this is even a valid comparison. I don't consider Mata a winger (he was forced to play out of position last year) but rather a number 10/AMF. Valencia is more of a out and out winger-he has pace, good control and excellent crossing abilities. Comparing apples and oranges.

@naman93: no, because saf experienced 25 EPL seasons, UCL seasons and more and wenger like 15 or something and valencia started playing in UCL 3 years ago

and Pep Guardiola experienced only 4 or 5 seasons

Valencia is strong and fast but silly, Mata is intelligent.

Silly?? explain.

http://www.youtube.com/watch?v=aWpItFt5niM&feature=related

He simply stated that his kick was both a pass and a shot. That's not silly at all - I would have said the same thing.

Valencia.

@Keisuke: No footballers are intelligent. They're footballers for a reason; they dropped out of school every single one.

@Norway ever heard of a guy named Frank Lampard? He attended Brentwood School, where he got 11 GCSEs, including an A in Latin. He got an IQ score higher than Rooney, Stevie G and Ashley Cole combined. Wenger was a footballer as well and he has a degree in Economics. Giroud contributed a year to college before moving with football. David James has a university degree. Graeme Le Saux was one of the most smartest footballers of our time. The Brazilian legend Socrates was qualified enough to be a doctor. Not all footballers are dumb...maybe Ashley Cole but not all.

@Keisuke lol that constitutes silly for him. Get a load of this guy lol! Even as a ManCity supporter I know Valencia's gonna do great things next season and this guy calls him silly for saying "50-50" lolz!! What a tool.

Mata is younger and has just played one season in EPL.  Valencia just had a very good season, at least a good second halve of the season. They play different roles for the team and their playing style is very different as well. Valencia is more of an old-school winger, no fancy dribbling and not much goal chances but very accurate crossing and very fast ball movement. Mata is more like a playmaker and much more of a fancy player with more goalscoring and trough-ball kind of assists. Overall I think Mata is better, although they are not very much alike Mata is 2 years younger and has not played in EPL as much as Valencia.

Mata's movement is more midfield, more inner runs. Valencia Owns the right wing with his own individual technique. Obviously I would choose the United player,not taking anything away from Mata/good player but Antonio came back from a broken ankle FFS lol. 
 

@Tuanis: I'm kind of surprised you didn't choose Valencia.