Re: Aikido Skeptic about to take the plunge.
Federation lines can be very misleading. The same is true with the cultural and/or historical narratives they use to define themselves (e.g. pre/post WW2). You should go and check out each dojo/instructor to see if it is something that you'd be interested in, etc. It is the only real way of knowing what's what. The need for this is something you seemed to have stumbled across already, only the difference between Aikido interpretations is still much more varied than you are currently/probably imagining.
For better or for worse, there is no one Aikido. There is only a bunch of aikidoka going around saying their Aikido is the one. My advice: Find the one Aikido that speaks to you and make it your own.
(fyi: Not sure I would say that Aikido is easier on the joints than BJJ - my experience points me toward the inverse being true. Finding an Aikido that is martially congruent with your other disciplines may prevent you from finding an Aikido that is easier on your body than BJJ. Perhaps you will have to make some sort of compromise in that regards.)
best of luck,