I've replaced the white paper with waxed paper and placed it in front of the coin, so now my little project is an actually diffuser, rather than a diffuse light reflector. I'm combined all three images of each coin for comparison. It's not a perfect comparison since the photos with the diffuser were taken a day later. Also, I rebalanced the color on the 2nd and 3rd photos so that the background hue was closer to the hue of the first photo. The first is the Athens tetradrachm.
Conclusion: the differences are subtle, and I can't say that any one setup is clearly the best. If you have a coin in good shape, just about any approach will make the coin look nice. If the coin is more worn or has rougher surfaces, maybe diffusing the sunlight through the wax paper offers a slight improvement. I think I can see a little more detail on the Pergamon and Maximian bronzes. But this may all boil down to personal preference and/or bang for the buck. Since I've already built my diffuser, I'll probably go ahead and use it. Is it worth anyone else's time to make one? I dunno.
I have to agree. Only subtle differences and hard to say which is best. I suppose it all comes down to which looks most like the coin in hand.
I agree - it's easy to dress up a coin with editing, and I think it should be shown instead as closely as possible to how it appears in hand, and preferably in natural light.