Mats, before you buy the 20, see if you can find one to try or look through the viewfinder with one mounted. I find the 20 almost too wide. I really like the view from the 24. The 20 looks almost unnatural to me.
I've bought from KEH, Adorama, B&H and my local camera store. My local store has a nice selection of used lenses and bodies and they usually price close to KEH so I'll buy from them because I can inspect it before buying.