I have numerous F3s, no direct experience of F4s, but I would say this latter is newer, considerably bulkier and heavier, has autofocus of course, is possibly a better prospect for extensive motor drive use, will cost more and be harder to find. The F3 does everything I ask of an SLR (including offering mirror lock-up for photomicroscopy, etc.) and can be found for little money (but very likely in need of a CLA). As with all Nikons, go for a cherished amateur-owned model, in pro hands a Niikon will soak up a lot of punishment but will wear out eventually.
Originally Posted by Mike Kennedy