I'm getting the impression that you are somehow thinking that I'm not agreeing with you, Jobo, because what you just said doesn't bear on what I said in relation tot he O/P.
To me your point is that bigger, stronger & faster people win more than smaller slower and weaker people, regardless of what they do to train themselves up. I can generally agree witht hat, except for your aforementioned outliers.
My point was that in my opinion a smaller, weaker person can make better progress, faster, towards being able to street defense skillset in BJJ for their initial term of practice than in aikido. That was it. Can you get there with aikido? I propose that you can, provided that you've got an real understanding in another couple sets of MA paradigms... but that takes way more time to get rounded out than the premise in the O/P. Some would stayt hat the "primary" SD art used wouldn't be aikido int hat sense, and arguments could be made either way and nobody would be either right or wrong. Kotegaeshi in aikido can be done as a wrist lock, a throwing technique or in jutsu fashion to directly destroy the wrist. Hapkido teaches the same technique, and the aim (with my training in HKD at least) was not to throw but to destroy the wrist (which is why the throws get "taken." Which is itself a whole another discussion that's going on in that other thread. So, when the bad guy swings and your hands fly up in the way and you end up with an arm, and then you slip and fall on the arm, which appens to land between you and the ground int he lock position and it goes "pop" were you doing aikido, or hapkido gravity-jutsu? Who knows and who cares. You got lucky, the technique fell into position and the effect took place. Does the label mnatter at that point, except to perhaps explain what happened later on to someone who was not there so they can grasp the events? I don't think so.
Back to O/P. BJJ better to start with, IMO.