I know will has cut some tunes using the Zoom G3 - I think it sounds great, personally. I've demo'd both and my ears tell me they're about even. The RP500 has more features, but didn't necessarily exceed the Zoom in terms of tone quality.
Also, don't take this as anything but information - here's website for the
Digitech RP500. The owner's manual will take care of just about any question you might have. To answer your question directly, you absolutely can edit effects settings.
So all this said, I would opt for the RP500 - the built-in expression pedal for wah/volume/Whammy makes it that much more compelling.