What do you think that the DA 40 will allow you to shoot that would be hard with either the A 50? I think the major advantages of the DA 40 are:
- stopped down bokeh (9 aperture blades).
- ergonomics for fast-moving / dynamic scenes
- fast autofocus
I'm not so sure that the 40 will be a major improvement over the 50 IF:
- you are only shooting landscapes
- you have time to focus (stationary targets)
- you are focused to infinity.
The DA 40 excels as a people lens IMHO. It's very responsive and renders fantastic details. I had a wonderful experience with it shooting my friend's new puppy recently, and realized that taking photos of him would have been near impossible with one of my 50's because he was so frenetic. I'm not sure you would see much difference with the DA 40 for purely landscapes. The DA 15 might be more your thing, if landscapes are the name of the game.
I also second the 28mm idea, I think it would pair well with EITHER the 40 or the 15. They are relatively inexpensive, and can take some very nice photos for "cropped landscapes". The utility of the 28mm is that of a "loose normal" (42mm equiv.) as opposed to a "tight normal" (60 mm equiv). I think the 28mm is a good landscape lens on digital, provided you pair it with something wider.
The kit lens at 28mm is pretty good, and the 28mm 2.8 prime is a good upgrade in image resolution. I would stick with the kit until you find you are frustrated with some aspect of the image quality.
A sample of my 28mm 2.8: