Islands has a slight storyline to it as skyblock is more black and white.
In islands you are introduced to your island with a minor story where your ship wrecked and you are stuck there. You are surrounded by water and it is poisonous. its generally harder to play as islands are smaller now. The player count is lower too. Its more quiet and peaceful. Though boats are fun in islands since you're surrounded by water. So building is easier to start expanding your islabd without fly and crouching.
In skyblock you are given an island and its up to your perception. The island is also much bigger and the player count is as well.
The games such are pretty much the same but islands is pirate themed and more of a role play like version of skyblock.
I personally prefer islands.