After (too?) much deliberation, I've ended up deciding to keep the 39mm. It was a really tough choice, and I'm still feeling some "returner's remorse" -- but here's how it all went down:
Basically, I felt, like a lot of other very helpful responders here, that both looked good. I was split between the fact that, at some angles, the 39mm looked too large and the 36mm looked too small. So it was kind of an even divide, but I figured that, since I had exceeded the return period, I could keep both for a little while and just decide. In the meantime, the 36mm was having some movement issues, so I had to send it back to the dealer for a warranty service. When it came back again and some issues still persisted, they gave me the option of sending it back for more service or just returning it for a full refund. But, I had to decide within 24 hours.
So, faced with that time-sensitive decision, I decided to just send back the 36mm for the return. So I've decided (rationalized?) that the 39mm still looks good. Does it wear big? Sure, but the bracelet is still visible on both sides of the case, giving it some proportion. And I have a few smaller, dressier watches -- so that need is met in my collection. As I see it, it boils down to a kind of philosophy, mentioned here, of whether the AT is a "sporty dress" watch or a "dressy sport" watch. 36 goes to the former, and 39 goes to the latter. And, I agree with a lot of posters that the 36mm has better dial proportions -- which may end up bringing me back to it at some point. During my deliberation, I also asked a few friends in person. All of my WUS friends (incidentally, all men) said that the 36mm looked better, while non-watch folks (incidentally, all women) liked the 39mm better.
One other factor that tipped the scales in favor of the 39mm is that, as mentioned in my first post, it was the watch that kicked off my collection; I have the full set, and even though I'm not the first owner, the warranty card shows that it was originally purchased one town over from my hometown -- even though the guy I bought it from was located on the other side of the country! Feels like some kind of kismet to me. It's also accompanied me for some pretty important moments over the last few years. Chalk it up to irrational sentimentality, but it is what it is.
At the end of the day, if the 36mm is meant to be, I'll come back to it. I've also toyed with trying out one of the 8500 series, which I found online and tried a used one at a local dealer (different color -- I would still get the blue with the teak lines). To me, though, the 0.7mm difference between the 38.5 and 39.2 is pretty minimal, and the thickness is still something that I would have to get past:
I also started toying with keeping some combination of the 39 and 36, or the 36 or 39 and the 38.5 8500... but at that point, figured I'd gone off the deep end, as having two essentially-identical versions of the same watch does seem a little crazy.
So, since I've made my final decision for now, here are two last pics of my 39 in this morning's light, giving its blue dial a chance to really shine: