On EL-Nikkors, 50mm f/2.8 is a 6 element lens where as f/4 is a 4 element lens.

f/2.8 version is better corrected for edge-to-edge sharpness and is considered a better lens. There's a debate on APUG how much difference there actually are between those two.

I only use the f/2.8 kind so I can't tell you personally.