I don't own either a Uno or mega2560. But from following the boards introduction (via their published pictures) if appears early one used crystals for both chips and later they changed to resonators for both chips. That may not be an accurate statement but it's what I'm going with. 
So which one you get if you order would depend on how long it's been in stock with the seller? And yes I would prefer crystals if I was buying, even though resonators might be 'good as needed' or not.
Lefty