It works completely different. The "global one" can use a Firefox built-in feature which in general provides better results. The "per tab" one does the trick by forcing some CSS rules to the website which can cause unsolvable issues with some pages.

So in general I would recommend the "global one" to people who want to change colors to "most websites" as it tends to work better with more websites. The "per tab one" could be better to disable the website color choice for just a few "bad websites" as it doesn't affect all the other tabs.