I'm going to have to say Cena on this one.
Exactly one of the points I planned on making. While wrestling came more naturally for Bret, seeing how Cena has developed over the last decade in to a great performer is admirable.
In comparing versatility, I think Cena has shown that he's able to work better with a more eclectic group of guys than Bret was. Bret is an all-time great in his department, but I struggle finding matches where he shines even when he's out of his comfort zone.
When comparing their "greatest matches", Cena has that category in my opinion, but to be fair I haven't watched Bret in a LONG time. If we could compile a definitive 10 best matches list for each guy, the comparison would work much better. I have to go rewatch Bret's greatest hits to see how they measure up to Cena's.