Thanks for giving me your point of view :)

I think I should point out a few more details (which may or may not change your view - I'd love to know either ways):

• I have been quite brutally honest with A, saying that people have told me to not work with A, and pointed out reasons. And A said "hmm, now that I think about it, the current situation does indeed make me feel like a risky choice; however, irrespective of whom you choose, I'd be happy to help out for advice (even if you decide on a completely different field). Also, if you are being advised by someone else, we could even consider a collaborative project. The only issue I have is with a co-advising situation -- I don't believe that would be ideal for you, since for your goals, you require to not be split b/w two people doing twice the work. Collabing on a project on the other hand, puts a shorter time commitment, so might even be advantageous (gives recos ; therefore collab with other unis too)". So yeah, I don't think A would be upset. :) However, this also makes me feel that A actually cares about me personally, and wants me to succeed (probably sees my success as a ticket out of his recent "slump" at the uni); though others (who left A) haven't felt this kind of vibe with A. I don't know :/
• My funding is not dependent too much on the prof, once I end up picking the advisor (dept guarantees that my PhD will be funded, as long as I am in "good standing" (there's some official definition and stuff, but in many years, there hasn't been any student who's lost funding)). The only advantage to B's awesome funding situation is that B's research group will be allowed by the dept to grow as much as B wants (profs who have lesser funding have caps on how many students they can take on -- worst case, if no funding, no new students; only older students can be advised). So yeah, I'm happy I'm at a place that has such a nice policy.
• I forgot to mention in my original post, C and B have a collaborative project that just started. Both are known to collaborate with other profs in the dept quite a lot as well. A on the other hand, collabs much lesser, but when A does collab, it is usually with top-tier profs at other univs. Does this make a difference?