Hi Conan, seems we both really like islands!
I've been to both Sardinia and Mallorca, and as already expressed, they are totally different. The areas of Sardinia with timeshares is pretty gorgeous physically, esp Arzachena where we were - good hiking around some amazing rock formations and gorgeous beach areas - but mostly it's a purpose-built resort (originally for the rich and famous), with little of old Europe, 'til you go back to prehistoric and then there are some sights. Our trip was either May or June and swimming was pleasant. We exchanged thru RCI into Hotel Residence Porto Piccolo where our unit had stunning water views in every direction and it was very satisfactory. The color of the water there was the most beautiful I have ever seen, aside from a trip to Croatia. Biggest downside and the reason I wouldn't return: you have to drive quite a distance from the resort areas to reach actual towns with old architecture and history.
Mallorca on the other hand has plenty of that. And you could island-hop to Ibiza or Menorca, neither of which I've been to but have considered both. (RCI doesn't have anything in Menorca, but DAE sometimes does and I believe II does.) My RCI exchange on Mallorca was to Cala Pi Club which I loved. Some of our meals in Palma de Mallorca were some of the best I've ever had. We saw a lot of the island in a week, but not all of it, and I'd consider going back. Our trip was in May and the water was absolutely freezing, I went swimming once - for a few minutes which was all I could stand.
We spent 3 nights at a b&b on Sicily, in Taormina. I'd absolutely go back to Sicily, good advice from Pompey Family on that, except for lack of many exchange options. This followed our exchange to Malta, which is very different from all of the above. There are fascinating prehistory and historical sights, and Europeans do go there for beach and sun, tho that wouldn't compel me.