The main advantage of an Xpan over cropping from 6x7 is that an Xpan functions more like a 35mm rangefinder camera, if that's what you're accustomed to. At one time it was also desirable if you wanted to shoot Kodachrome, but that's not an issue anymore, alas.
An advantage of cropping from 6x7 is that you have more flexibility about the format shape and where to put the horizon in your panoramic images--like having a view camera with rise/fall (or shift when you shoot vertical cropped panos).
Best of both worlds: Mamiya 7II.
Like David Brown, I also have a swing-lens camera (Noblex 150), which is indeed a totally different animal, as he says, but for the right scene, it's perfect.