They are really two very different cameras. Optically, the Xpan is far better and it can be focused. The Xpan has better control over exposure as well. The Widelux has a greater field of view, no issues with vignetting, and a faster lens - the Xpan lenses (45mm and 30mm) are really not designed to be used wide open. One has a flat film plane and suffers from the wide-angle effect, the Widelux has a curved film plane which gives it its unique qualities.
I use a Widelux and a 6x12 panoramic camera. The 6x12 flat-film plane camera is more versatile, and I would imagine the same for the Xpan. The Widelux and swing lens cameras are unique and do things normal cameras cannot. While my 6x12 is a workhorse, I keep the Widelux for its special properties. I would say an Xpan and Widelux would make an exellent combination, but one is not really a replacement for the other. If I was limit to one, my tastes would go to an Xpan as it would be more versatile - Jeff Bridges may not agree.