PPSSPP is the PSP emulator
PPSSPP plays PSP games and it plays them well. Upscaling, cheats, and ad-hoc multiplayer are features of that project. It is free, open source, and the correct download if the backlog is only PSP.
RetroApp does not out-emulate PPSSPP on PSP. A launcher that claims otherwise is advertising. The reason to open RetroApp is the other twenty consoles and the people you play them with.
What you lose if you only keep PPSSPP
A PSP ISO library has no SNES, no PS1, no GBA. PPSSPP will not scrape those, will not keep their save states, and will not put a friend into a 16-bit game. That is not a flaw. It is a single-system emulator.
RetroApp's scan, covers, and cloud saves apply across the library. PSP can live in that library beside everything else. The high-end PSP upscale and CWCheat stay reasons to open PPSSPP itself when you care about that one game's picture or a specific cheat.
Multiplayer
PPSSPP's ad-hoc support is for PSP games that had local wireless. RetroApp's sessions are for the retro library, not a reimplementation of PSP ad-hoc. Use the tool that matches the game you are actually launching.
If you came from a frontend comparison, the LaunchBox, Playnite, and EmulationStation pages are the closer match. Dolphin and PPSSPP are here so the table does not pretend a launcher replaces a great single-system emulator.
Download RetroApp · BIOS guide · Save states · Best retro launcher · vs LaunchBox · vs Playnite · vs EmulationStation
RetroApp vs PPSSPP is a launcher comparison. Specialized emulators stay better at their one console. The table above is the feature split. The text is the part that should decide the install.