I prefer Island Living personally despite it lacking in activities because I love Sulani and the island lifestyle in general.
Snowy Escape has more activities and it's fairly decent expansion by Sims 4 standards but also if you're looking for a lot in the pack then it's probably going to disappoint. Nothing is really as it seems from watching the trailer.
Skiing is pretty much just an skill based animation which I'm fine with but there's only so many times you can watch it before it gets old. Same for Snowboarding.
The mountain excursion is a timed event that has no freedom for the player to do anything other than what the game tells you to do.
Hiking is just another long animation that doesn't really allow for any freedom of choice once again. If you attempt to interact with anything along the way it cancels the hike. This could be a glitch but I'm not sure.
Lifestyles are interesting in a rather unimpressive way. They seem to do not much but make sims miserable.
As always the CAS/Build&Buy items are great. If you're a builder you will maybe like it for new items.
Sentiments are great but since they come with the base game they aren't really a good reason to buy the pack for.