I, being a fan of both gamemodes, can tell you that they're nothing like each other at all, and anybody that says so has evidently never played both games before.
Skyblock is at first, a tedious gamemode in which you're left on a chunk of rock in the sky with limited space to build, breathe or set up farms, and of course with the permanent risk of falling into the void. It turns into a grind-fest, having to wait for trees to grow so that you can get enough wood to make another stone pickaxe whilst you mine cobblestone from your dull cobblestone generator for 40 minutes to build a house. However, once you've expanded enough then the world is yours - you can get animals, make farms, build skyscrapers and mob farms and it basically becomes animal crossing, but with brutality towards animals.
Survival island however is difficult from start to finish, with the players being stranded on an island and locked on hard difficulty for added intensity. Sure, blocks are easier to obtain and you get the privilege of gathering your own ores rather than purchasing them from a shop plugin, but due to the far-out ocean biome separating you from the mainland, your island is a giant green mob spawner, with zombies, spiders, creepers and endermen attacking you relentlessly. The aim is to defeat the bosses with limited resources, and that's exactly what you have; limited resources. You have to grind for food throughout the entire game, and there's no room for luxury due to the intimidating mob spawn rate.
However due to the size of this server, i doubt that a survival island server will ever be possible, since the gamemode would have to be instanced to ensure that you don't get raided by other players. This would take up a lot of memory since the worlds have to be very large to ensure that each island has a large enough ocean, an adequate amount of mines and cave systems, and an island big enough to support up to 4 players. Because of how many players there are on the server daily, you'll have hundreds of islands being generated constantly, and these worlds cannot be small as otherwise it may as well just be skyblock with mining.
If you want to play survival island, then downloading the map would probably be your best bet. Of course, there's always the suggestion of resetting the game every month, but then nobody would play it knowing that their hard work is going to be lost in a few weeks time.
