Modpack workflow
Prism and GDLauncher both browse and install CurseForge packs from inside the launcher, so the everyday experience is similar. The friction lives at the edges: when a mod author has opted out of third-party API access for their file, Prism asks you to click through each blocked link and download those files manually in a browser. GDLauncher's CurseForge partnership fetches those files directly, so installs stay one-click even when packs include blocked mods.
Modrinth packs work the same in both launchers, browse from inside the app and install in one click.
UI and discovery
Prism's Qt-based UI is functional but utilitarian; the main view is a list of instances. GDLauncher's UI is built specifically for finding and managing modpacks, with a built-in browser, instance grouping, drag-and-drop reordering, and visual cards. Subjective, but worth a look at screenshots.
Cloud Instance Sharing
GDLauncher has one-click Cloud Instance Sharing: paste a code, get the exact same setup. Prism has instance export/import via files, which works but isn't quite as friction-free for sharing with friends.
The verdict
Prism is excellent if you want a barebones, lightweight launcher and don't mind doing more work yourself for modpacks. GDLauncher is for players who want one-click installs from CurseForge and Modrinth, Cloud Instance Sharing, and built-in server management without leaving the app. If you're new to modded Minecraft or value polish over minimalism, GDLauncher is the easier path.