If the slab is present, then white hides it better. If the slab is cropped out, I prefer a black background, although in some contexts (web sites with white backgrounds), white looks cleaner.
With the image on black I think the coin looks better, but I don't like the black. Crop that pic and put it on white.
Most definitely the black background. The white seems to appear washed out while the black provides nice contrast IMO.
I tried my best to replicate everything (such as lighting and angles) as similar as possible but I think you might be right. The lighting is from the sun (via windows ), and although there wasn't a long time between photos it's possible that clouds changed the amount of light. It was cloudy earlier today when I took the photos.
Wow, nice! May I ask how do you crop circles? (haa, that actually sounds like I'm saying crop circles. UFOs! )
This may depend on the program one uses, but with Photoshop it's easily done using the elliptical marquee tool.
Photoscape. Freeware........allows you to crop in the round. My below link is a bit 'dated', but it should set you on the mark. https://www.cointalk.com/threads/photo-editing-and-presentation-with-photoscape.175157/