Actually, the Xpan distorts and the Widelux does not. But the Xpan "looks" natural.
As Pablito says, they are very different beasts. I have a Widelux and a Horseman SW612. I have found the flat field cameras like the Xpan and Horseman are more versatile. Naturally you have a vignetting problem with those cameras, but that can be cured by stopping down and using a center filter.
As far as lenses, the Widelux has a 26mm lens, but with a curved film plane giving a diagonal field of view of 140+ degrees. The Xpan 30mm has about a 90 degree FOV. The Xpan suffers from the wide-angle effect - round objects at the edge of the field being "stretched" away from the optical axis. The Widelux does not have this problem. But the Xpan has three interchaneable lenses.
The Widelux drum needs to move consistantly or banding will result. The F8 model is supposed to be the best in this regard. Also, you need to set aperture and shutter speeds BEFORE winding on or you can break the camera. Never force the shutter speeds. The Xpan is worry free.