The yellowing is generally believed to be caused by a over abundance of a chemical additive that acts as a fire retardant. What happens is essentially hastened oxidization. Things like excessive smoke, exposure to UV light, etc can exacerbate the problem. It is less common with the SNES mini.
It's assumed that the plastic formula contained a more balanced mixture with less of the fire retardant. It's not only an issue with the color, but the plastic is actually experiencing chemical breakdown. You might notice how extremely brittle the heavily yellowed SNES systems are. The "fixes" out there are just a bandaid, and I can't help but think that they are doing even more damage to the plastic, causing it to become even more brittle and likely to yellow again. A clear coat will help a lot, since exposure to oxygen is what really kick-starts the chemical reaction. That's why you can pop them open and the insides are often a lot closer to the original color than the outside.
Buy the ugly mini, then grab a wiggy shell when I finally get them to market
