The F3 enjoyed a longer and more popular sales run. If you don't want a motor driven SLR, then the F3 is a smaller and more compact set-up than an F4S. The biggest downside to an F3 (for me) is the non-standard flash shoe. There is a wealth of quite good Nikon lighting gear, and without an adapter you are restricted to much older Speedlights with the F3.
The F4S probably has one of the easiest to read viewfinders in the industry. When light levels drop, simply turn on the assist light for the displays. It also has a great range of capabilities and options, though some of the accessory items cost nearly as much as a good used body. I have and use often the waist level viewfinder for low angle shots. On a film flatness comparison, the back on the F4S is slightly different than the F3, which in theory might allow better film flatness.
I think though it really comes down to trying one in hand. The F4S is definitely not a light camera, though the grip set-up gives it a good balance. Ideally you would handle both cameras, and decide which one is the least distracting.
Gordon Moat Photography