if you are planning to get the F5, then get the AF 20mm. while I love the F5 I save my manual focus lenses for my F4 and FE2. you lose matrix metering with the F5 and MF lenses. the matrix meter in the F5 is its strong point and does wonders with slide film. The 20mm AF D lens is great on the F5. its size is great and easily fits in your jeans pocket.
take a hard look at the 50mm 1.8 D lens. The best $115 I spent on a lens.