Casio is phasing out electroluminescent backlighting and going back to a single LED, so I would buy an F105 while you still can. I've heard it said it was to improve battery life and longevity but I've never had a problem with either one.
If they update F91W to include better backlight, then good riddance. Their newer models (like A700W) have single-LED backlight, and it's excellent (probably better than EL, but I haven't used EL in a while). The watch is very thin too, unlike EL models.
