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?