Which type of a server is better?

A skyblock modded server is going to be far easier to manage than an open-world survival modded server.
If you're going to host it for just your friends, either will be around the same...
But if you're looking to open it to the public, managing a modded Skyblock server is going to be far easier than a Survival Modded server, because you won't need to worry about griefing or anything along those lines, the plugin will sort that out for you, and you won't need admins and moderators to manage the server,