The girlfriend likes coop games the most, so we tend to play a LOT of pandemic and forbidden island/desert
They are all very similar but end up playing out differently in the end imo.
Pandemic and island share the "get 4/5 of this color card to get this item/cure from this specific place" idea.
Pandemic and desert share the "sand/disease cubes pile up on parts of the board, get them off before it gets to much" idea. This, I feel, is different from island, which is a simple underwater/sunk mechanism.
Desert has its own thing going for it with the hidden objects mechanism, making you explore a location, and then find the other matching location of that color to pinpoint where the object will appear. This and the water mechanic, I think, sets it apart from Pandemic enough to make it still interesting if you already own and love pandemic.
We have played a lot more desert and pandemic, but I think I prefer island. I like the urgency created when something goes underwater, since you know that it can now simply sink into the ocean if you dont get there. Pandemic has outbreaks, and desert has stacking tiles that prevent you from moving, and both of them will end if cubes or sand tiles run out, but it always seemed a bit easier to me. You can not get stranded on a tile away from everybody else in pandemic/desert.
tl;dr: Forbidden island is the best.