Having been a kayak and open canoe coach in the past, I disagree completely with your assertion that a kayaker can become a proficient paddler
"Completely" suggests there's nothing there at all, which I think is taking things perhaps a little too far? Basic proficiency is easier in a typical case in a kayak becuase the control of the craft is symmetrical, so once you know how to turn one way you know how to turn the other, and going in a straight line forwards or backwards is relatievly straightforward. That's simply not true in a canoe where there is a much greater range of strokes coupled with assymmetric control to make the thing go where you want it. And that's without the overhead of communicating between bow and stern about what's needed if you're two up. So that's why I think that for the typical case.
Different learning rates don't obviously affect which one is easier to grasp, just the speed at which it is grasped. Being unreceptive to tuition tends to make more difficult things increasingly more difficult, so that again favours basic control in a kayak. Being terrified of moving water doesn't really affect how one learns canoe and kayak strokes on still water. I prefer the poosition and single balde in a canoe, but I still find kayaking technically easier...
I am not saying kayaks are better or preferable (I prefer canoes myself), but they really aren't as easy to manoeuvre, which is why I think basic proficiency is harder. My underlying point is if you've got a limited time at it (seems to be entirely likely here) and have a particular non-trivial goal ("I want to shoot an easy rapid on my own!", say, rather than "I just want to do some paddling stuff, anything will do") chances are you've got a better chance of getting where you want in a kayak. Personally I prefer a canoe, as I find the control more satisfying in part because it requires more skill. Almost anyone can put a kayak in a staright(ish) line for 20m on a flat lake, the same is not true of a canoe.
Pete.