As far as I can tell there are zero advantages of a B-7 over a C-7. I dont know that I buy the air retention comment. I figured anyone using the two different valves was trying to use up their stock. Most every clean C-7 will hold air great for days without the cap on, although the cap should always be used to provide the final seal (on both valves), keep grit out, water out and to protect the innards. If grit gets inside the valve on any model, it will leak, period. Both valves will hold air great if they are clean and tight. I just got off of Cataract and I pumped my boat with C-7s up at the put in and never put another stroke into it over 7 days. Not sure what else you could ask from a valve.
The fact that the guts are on the outside portion of the C-7 is enough reason to never use any other valve in my opinion, until the D-7's come out of course. The A-7's and B-7's are a bitch to service. The lower profile cap is just an added bonus, albeit a nice one.
The blasted A-6 PRV is the valve that leaks like a sieve. 90% of boats that come into the shop have a leaking PRV. If someone would figure out a better design for a PRV that threads into an A-6 back you could sell it to every rafter on the planet.