An xpan is a fine camera. But (as has been stated) it is not unlike a cropped medium format negative. Think of it this way: an xpan with a 45mm will look essentially the same as a 45mm (or 50) lens on a hasselblad or a tlr if you simply crop the 120 negative off the top and bottom so that it's only 1 inch high (by 2.25 inches wide). That 1 x 2.25 is pretty close to the negative size of both the xpan and the Horizon. The xpan with a 45mm lens gives a field of view around 70-75 degrees (estimate). I can get pretty close to the same "image" by using my 55mm lens on my RB67 and cropping.
HOWEVER, the Horizon works entirely differently. For one thing, it gives a field of view of 120 degrees! This is achieved by a 28mm lens and a film gate in the back of the camera that is NOT flat, it is curved. This is so, as the lens rotates, the distance from the lens to the slice of film being exposed at that moment stays the same. This causes objects at the ends of each frame to be both optically and literally farther from the camera then things in the middle of the frame. Example, if you wanted to take a picture of a group of people all standing next to each other, you would have to arrange them in a semi-circle in front of the camera where each person would be the same distance from the camera. Otherwise, the people in the center would be the tallest, with diminishing height progressing to each end. So, yes, a brick wall (or almost anything else, including a not perfectly centered horizon) will appear curved in the image.
Landscapes can be achieved if you get the horizon (of the Earth, not the camera) level and centered in the camera. This is, to some, a limitation; but when used properly, can make spectacular images.
Think of it as the Horizon camera being a true panoramic, while the xpan is simply just wide angle.
Clear as mud?