If you are really going for the best IQ - image quality, then like lytrytyr posted, do not overlook stitching. Wide angle lenses are not necessarily just for landscapes. They encompass a large field of view, which contains a significant amount of foreground. Many times you want to bypass or overshoot the foreground for the landscape - thus a longer focal length will be perfect. The longer focal length brings better definition to each pixel. Even though I really liked the 12-24 and thought it was very sharp, shooting and stitching the 31 Ltd killed it. The difference in IQ with the smaller representation of area for each pixel can't be matched.
If you look at most of the commercial landscapes, you will see that they used something between 24/28 to 35mm for a focal length. Why - lack of distortion and great pixel definition. You can use any lens for a landscape - even telephotos (which flatten the view quite a bit). There are reasons for using various focal lengths in lenses for specific views and composure. For wide angle, there are some applications where you can only get a single shot due to motion or whatever.