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If you are only going to do server side stuff, you should probably use the Sponge API since it has a lot more helpful systems like the Data API or the cause tracker. Forge is bare-bones, do-it-all-yourself. And if you find yourself needing Minecraft internals, or even the Forge API, you can still use them from within Sponge. Sponge also makes a bunch of optimizations, so it can be faster than vanilla in some cases. And to answer your question about permissions, neither is ‘more secure’ but Forge doesn’t actually have a ‘permission’ system whereas Sponge does (Forge permissions are hacked in by checking if a player can run a certain command, where the command is the permission node).
Sponge also has a smaller chance to break through minecraft versions
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