I can thoroughly recommend the Pentax M f2.8 28mm, another good option is the Tamron f2.5 28mm Adaptall 2 lens. I have both and they are excellent lenses.
From personal experience f3.5 lenses are a little restrictive in low light situations, the extra stop makes a huge differance.
A 28mm is immensely practical as a second lens and compliments a 50mm very well. I used a 24mm lens and found it was too great a jump compared to a standard lens, and wanted something in between. So I now use 50mm, 28mm & 17mm which is a great combination.
Ian