Some of this has been discussed already but the xpan can't shoot an image thats 140 degrees wide, and if it could the corners would be pretty distorted. The swing lense Horizont doesn't have to use a very wide angle lens to shoot that wide since it is moving on a curved film plane. Also it uses the best part of a lens through a vertical slit, not the edges, so it doesn't need a quality optic as much to get good sharp pics. I did a diagram of this relating to a rotating pan camera, but it would still be the same for a swing lens camera.
http://www.jamieyoungphoto.com/roundshot%201.html
I borrowed a widelux at one time and loved it. Maybe i'll get a perfekt one of these days. Thanks all for the reviews