I second the above. Seasons fixes a big problem with the immersion of the game and that's the lack of feeling that time is actually passing, beyond day and night cycles. It's got something for everyone. It doesn't matter if you prefer realisim, occults, you like or dislike family gameplay, you're a screenshotter, you're a builder, etc. You'll like Seasons.
Island Living, on the other hand... the only thing I liked about it is the world. I screenshot often and use it as a favorite vacation spot for my Sims. That's it. Mermaids are uninteresting and not nearly as fleshed out as the other occults, scuba diving is essentially a rabbit hole when in TS3, you could actually play your Sim underwater and explore the reef. The gameplay just consists of cleaning up the island, which is basically sending your Sim around the island, picking up trash, and doing that over and over again. Island Living accomplishes giving more location and build variety into your game and little else, in my opinion.
Seasons accomplishes enriching the entire game whereas Island Living suffers from a big problem a lot of packs have. The majority of its gameplay, gameplay that's much more shallow than the ocean the island is on, is locked to its singular world. Which, if this game had open worlds, wouldn't necessarily be a bad thing, but it doesn't. So it is.
So if you have plenty of worlds and want a pack to improve all of them, I'd say Seasons. If you're sick of the towns, suburbs, cities, etc and want something different, then... I guess Island Living.
