The FF was the first divers watch with a movable Bezel and the other specifics. Rolex Submariner based their design off the FF.. If you want the "Original" dive watch, then go with the FF.
Some advantages the FF (last years 40mm model) has over the Sub: FF has better lumination, AR coated on the inside Crystal, and a movement that has twice the power reserve, and if you go with the SS bracelet version - considered to be one of the best made SS bracelets and clasp in the watch industry. The newer model FF is 45mm, much larger, but also has many upgrades that are really outstanding.
Here are some links for your review:
http://www.fiftyfathoms.net/