Albert Smith , Jun 23, 2005; 10:53 a.m.
I have used every 50mm Nikkor. I have the 50mm f/1.8 AIS Nikkor (larger tube) and find it to be a very fine lens. The f/2.0 Nikkor is also fine optically. For most situations, the difference between f/1.8 and f/2.0 is so close that you can ignore that fraction of a stop. The finder is not brighter with the faster lens, at least not so much that you can see it with the naked eye, and there is not a sigificant DOF difference between these two apertures (take the f/1.8 lens and set it to f/2.0 and then pop the DOF preview lever if you don't believe these points), so one lens does not have a leap in image quality potential over the other.
I chose to go with the f/1.8 lens because it had better coatings than my f/2.0 lenses (had about 5 of these). I shoot slides a lot, and I felt that the contrast was better with my 50mm f/1.8 AIS lens.
I also have a fine 50mm f/1.4 AI Nikkor for times when f/1.4 is an advantage, but I use my f/1.8 AIS lens for technical photography, when I need sharpness, contrast and correction for distortion, and have never been let down by it.