Too early to tell. WAY too early to tell. If I was to take a guess, I would say Mysterio simply because Sin Cara has a lot to live up to. Mysterio is such a legendary figure that it's going to be hard to eclipse him. That, and the fact that they're both small guys who wear masks probably means that the fans are always going to see Sin Cara as a duplicate of Rey or knockoff version, much in the same way Swagger is considered an Angle knockoff.
Also, it needs to be taken into account that Sin Cara cannot speak English, and even though I'm sure he'll learn eventually, for the time being, it could be a detriment to putting him on the path to eclipsing Rey. I know Rey's a shit promo, but Sin Cara not speaking English period may result in some fans not being able to warm up to him the way they would otherwise. People will have nothing to attach themselves to about his character, he'll just be a guy who does stunts.
It's also worth pointing out that Rey Mysterio's look is absolutely fucking phenomenal for an underdog babyface. He has the perfect sympathetic, underdog hero look. Because nobody can see Sin Cara's face whatsoever, we'll never get to see his expressions during a match as he takes bumps, as he gains momentum, if he feels like he's in trouble. This is a big part in connecting with the audience and Rey is able to do this perfectly. His expressions are top notch and always add to what's going on.
However, I suppose it can be done, but it's going to be difficult. I will say it can't hurt Sin Cara that he's bigger than Rey...