Mata has several goals against Liverpool, Arsenal and City to his name - and I can recall excellent performances from him against Spurs and Chelsea too, so I'm not sure the big game argument stands up.
Fellaini has also scored several goals against top six too, I recall goals against Spurs, Chelsea, City, and Arsenal off the top of my head. I don't think Mata has achieved something extraordinary in PL compared to Fellaini.
In cup competitions Fellaini has had far more of an impact mainly in that Europa League run which I mentioned in my earlier post.
It's not really about opinion though, check my earlier post in the thread which shows Matas important contributions in every cup win we've had post Fergie.
Mata has had at worst a similar impact in the cups to Fellaini, I think people are just saying Fellaini contributed more in cups going off memory rather than actually looking it up.
Big performances, important moments just stand out without looking it up. People remember Mata's brace at Anfield because it came at a very crucial stage of the season, same applies for his FA Cup equalizer.
With Fellaini I recall more of these moments, scoring in a 4-2 win over City, 3-0 vs Spurs, 2-1 win over Arsenal. Scorer in FA Cup SF, Winner in EL SF, MOM in EL Final.