Bukkit Plugins that are disabled during onEnable() are not what we would call “pure” Bukkit plugins. They use internal Mojang code which is version specific. A plugin uses Bukkit only will not break from version to version. In theory, this is true for Sponge as well, but the API is new and is changing a lot, though it is getting much more stable.
Sponge is not purely open source either. It is just as open source as Spigot or CraftBukkit. It still uses Minecraft code. Glowstone does not. But Glowstone is not complete, so unless you’re willing to experiment with Glowstone, I would recommend PaperSpigot, which is a fork of Spigot with bug fixes and performance improvements.