Even though the 28mm f2 has a bit more barrel shaped distortion then the 28 f2,8 Ai-S (do pick up the Ai-S rather then the Ai version. Worlds of difference between the two!) I personally preferred the distorted view of my 28mm f2 Nikkor-NC over that of the 28mm f2,8 in most cases. The extra stop of background seperation and the image sharpness and clarity really make the f2 version an excellent choice.
I do not quite see where the 20 f2,8 comes into the equation. It is such a different lens. With the 20's either pick up the latest or oldest. The 20mm f2,8 Ai-S has the sharpest center. The 20mm f3,5 Nikkor-UD the best overall sharpness with better corners but slightly less sharp center. Merits of the 20mm f4 and 20mm f3,5 Ai are merely small size and 52mm filter size but none optically. (ok, maybe flare resistance on the 20 f3,5 Ai)