Depends on the PC, but generally speaking PCSX2 will probably always be better than the PS4's PS2 emulator. This is because PCSX2 is designed to run all games and be easily configurable, whereas the PS4 uses "different" PS2 emulators that are slightly tweaked depending on the base game you injected the new game into, each one configured specifically to run its base game as well as possible.
IIRC, someone said there were over 30 different PS2 "emulators" available (though that was a while ago, I know there are 50 some games now so maybe even more?) and performance for your injected game could widely vary depending on which one you use.