I mostly shoot with a 50 mm lens (on my DSLR with my 35 mm f1.8, on my F100 with a 50 mm f1.8 and on my Bronica 6x6 with a 80 mm f2.8), which I love but I feel I need to learn something new pretty soon. Like a new perspective, literally

So I'm out looking for some more wide-angled prime, preferably in the 20-28 mm segment. It would be good to be able to use it on my DSLR (D7000) as a approx 30-35 mm (so 20-24 mm) but having a wide lens would be nice for my F100 as well.

I've googled this to some extent and there seems to be multiple options, but I'd like to hear your recommendations. I'd prefer it the lens was small, semi-large aperture (f4 is good enough) and not very expensive, but I guess I can (at best) pick two of these.