If you are flying AF almost exclusively and can qualify for it, the hard benefits of Platinum Ultimate (4x one class upgrade certificates that confirm into most revenue fare classes, nominate a companion to FB Platinum) may provide more value for you than Diamond (4x GUCs that confirm premium economy and waitlist for business day-of-departure, plus two other choice benefits).
You'll also almost always get the best soft benefits on the carrier that your FF status comes from.
Ultimate will be tough to hit most years. Also, GUCs have one awesome feature over Ultimate Upgrades in that GUCs are one-way with no regard to segments or distance. So YUL-CDG-SIN only uses 1 GUC. The Ultimate certs only allow for 1 long-haul segment. So YUL-CDG-SIN would cost 2 upgrade certs. If they changed them to one-way, I would push hard for Ultimate. Also, GUCs clear from Prem Econ to Z-Business on DL/KL/AF and G on VS. So they are the same there.
We have a whole thread over on Flying Blue about DL Elites switching to FB. More discussion on this topic would be welcome over there!