Is there a benefit to using a "real" crystal, as opposed to a resonator? (running with the internal does not impress me much, as timing would and could be important depending on the application). Is one more accurate over the other? or is it just down to cost...
I don't think there is an issue with accuracy, but if I had to guess, the crystal would be the more accurate of the two. I think it mainly comes down to parts counts and probably availability, as well as parts placement on the PCB. For a custom PCB, you can probably get a smaller size using a resonator than a crystal+caps. However, I am not sure that a resonator has a standard 0.1 inch lead spacing, and so might not be easy to fit on a stripboard or the like for a simple prototype (this isn't an issue with a custom PCB, of course).