I know a couple guys with the F&D...it is a REALLY nice yak. They have monster room and storage, not to mention stability out the wazoo. its very easy to stand and fish or pole in a F&D. You can too in the Nav...as I can my Redfish, but given the choice Id go fish and dive all day. They are pretty comparable yaks, but taking into account the width the 31" nav vs the 36" F and D the main difference between the two is room and stability.
Not to mention the monster compartment in the floor of the F and D...you can put a small army in there...
My suggestion is to go paddle them both and decide what YOU like best...