If you want the best possible image quality for landscapes you should consider using a somewhat longer focal length lens, which can then be used to produce wide-angle photos by stitching together multiple images. With software like Autopano Pro you can get great results with ridiculously little effort. It's amazing how easy this is to do, and you get medium-format quality on a budget! Of course this only really matters for very large prints. For smaller prints, single-image APS-C photos with any decent lens will be fine.
Stitching software has improved a lot in recent years. It has not only become easier (from start to finish), but the stitch quality has improved. Here's a link to my favorite stitching program. While it gives you a great deal of control if you want to fiddle, I find that the default settings work great in most cases, saving time and effort. The program is quite fast, even on my 7-year old Linux box:
Autopano Pro | Panorama software for Windows, Mac, Linux | Kolor
With the 20-40mm zoom you could get very wide angle shots by stitching together a few handheld images at the 20mm setting (shot vertically), or for best quality you could set up on a tripod and go crazy with the lens set at the 40mm setting, shooting a lot of images. I often use the DA40 to shoot wide angle photos. It used to be the case that you needed a special rig to rotate the lens at its optical center, but I find that this is no longer necessary for landscapes given the increased sophistication of the stitching software. Of course for closeup work proper alignment is still necessary.
Dan