Two platforms, both worth using
CurseForge and Modrinth are where you get Minecraft mods, modpacks, shaders, and resource packs. Both are good. GDLauncher searches both simultaneously, so you don't really have to choose. But here's how they differ.
CurseForge
CurseForge has been around since 2006. It has the biggest library, especially for older mods and established content. If you're looking for something from the 1.7.10 or 1.12.2 era, it's probably only on CurseForge. It's also the only platform with Minecraft world downloads.
The downsides: the website has ads and can feel slow. Third-party launchers need special permission to access downloads, and some mod authors restrict third-party distribution. It's owned by Overwolf, which some people have opinions about.
Modrinth
Modrinth is newer and built by people who wanted something better. The website is fast, clean, and ad-free. The API is open, so any launcher can use it. The search is excellent. Many players and mod authors prefer its approach.
The downside is a smaller library. Modrinth is growing fast, but older mods often aren't there. And unlike CurseForge, Modrinth doesn't host world downloads.
What's where?
Most popular current mods are on both platforms. Sodium, Create, JEI, the big modpacks, shader packs, resource packs. Authors often upload to both to reach more people.
CurseForge exclusives tend to be older mods, FTB packs, and anything that predates Modrinth. Modrinth has some newer mods from authors who prefer its policies, plus a strong open-source modding ecosystem.
For mod authors
CurseForge pays based on downloads and ad revenue. Bigger audience, more downloads. Modrinth has a direct creator program and a better developer experience. Many authors publish on both platforms to maximize reach and revenue.
The practical answer
Use both. GDLauncher's mod browser searches both platforms at once. You get CurseForge's library plus Modrinth's without having to think about it. When the same mod is on both, pick whichever you like. Updates work regardless of where you installed from.
If you have to pick one: CurseForge if you need older content or world downloads. Modrinth if you care about open source, hate ads, or just prefer the interface.