Petrov. Just.
Both are classy midfielders with good vision, both rarely give the ball away, both read the game well.
Petrov works harder though, much better tackler, and gives his all for Villa. Barry is more likely to produce something special out of nothing, but this season in particular has dissappeared in way too many games for my liking.
As others have said before, I don't think they're particularly suited as partners in a 4-4-2.