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All right, I'll state the obvious, just for the heck of it, to get the basics out of the way:
1. It depends on the composition of the painting or photo. No one aspect ratio does it all for all subjects.
2. Customers almost never buy my 15:1 super-wide panoramics. You might as well say never, since nobody has yet.
3. The golden mean ratio is 1.618, not 1.5
4. Diversity is good. This especially applies to art.
5. If it sells, it's good. What do the sales tell you?
6. You tend to see what you shoot. If you shoot 3:2, that's what you tend to see. If you shot 1:1, that's what you tend to see. If you have an indexing panoramic head, guess what?
Never really thought much about it. I shoot whatever is native to my camera. I often crop for composition without even considering the aspect ratio. I also shoot panoramas and stitch them with an eye toward the overall composition--again without really considering aspect ratio. And I have sold panoramas, as well as other sizes. In fact, one of the main reasons I chose FAA for POD is because they accommodate custom sizes. All the other places I looked at made you crop your work to fit in their boxes. I don't like fitting my images into someone elses boxes, lol. Maybe you're overthinking it?
The native ratio would be the one using all the pixels the sensor will record. I'd go with that; anything else would be throwing away options soon as you take the shot.
Per Wikipedia the native res of your camera is 3888 × 2592, or a 3:2 ratio. The rest, or just about any other you can think of up to 3888:1 can be achieved by cropping, and you'll get the best compositional flexibility by doing that in post rather than in-camera.
The 4:3 ratio has literally been around since the 1940s, or so, whereas the 16:9 ratio has long been a favourite of today's movie producers, as they literally wanted to make their movies much bigger to attract the public mainstream. Many people who grew up in the 50s, 60s etc. can easily relate to the 4:3 from their childhood teli shows.
The 4:3 will produce a much more box-like picture, and the 16:9 is equated to more of a widescreen image.