They're so similar in the ring. Yes, Rock is a better seller. But he's quite goofy at that as well. They have the same style in the ring, and pretty much the same resume when it comes to quality matches. I will say that Cena can get better matches out of lesser opponents than the Rock can, has has during his career.
The win-loss thing is irrelevant. If anything, it makes it tougher for Cena to do a good job in the ring. It's a different era now than it was 12 years ago. If you put Cena in the ring back then, he's going to be comparable to the Rock. If you put Rock in the ring now, he's going to be comparable to Cena.
Both guys are where they are because of their charisma and because they understand how to work a match. But I do think that Cena's resume is far more versatile than the Rock's, who if you take out Triple H and Stone Cold his resume looks quite bland (I'm talking only about quality matches here). Some of it is longevity, but Cena has had more great matches out of more wrestlers, and again, I think Cena is easily better than the Rock was at getting good matches out of bad wrestlers.
At the end of the day, they're pretty much even when it comes in-ring quality.