Mobi vs azw3 & KFX (which is similar to epub)
Mobi (azw): no embedded fonts, only a few built-in fonts, no CSS, just inline styles in HTML3. A few not very useful mobi extensions. There are images, but they might not be sensibly sized.
KF8/azw3: Can have embedded fonts and more built in fonts referenced. Subset of CSS3 and HTML5 and some Amazon extensions. Might have Media queries that work. Unpacks or converts well to epub2,
It depends on the content and formatting. For a simple novel all in one body font and simple headings it likely doesn't make much difference.
Occasionally an old ebook title is only delivered in mobi/azw direct on a Paperwhite 3 or Download & Transfer.
I'd download anything not downloaded
More recent years uploads are usually epub2, which converts almost exactly to azw3. I suspect KFX is a repacked binary/compiled azw3 with different rendering and encryption. The mobi upload for ordinary ebooks is gone ages ago.