Try yourself
It doesn't matter (for PSP ones*) - use whichever format you prefer (which for me would be pkg because I love having everything in the proper official UI)
* in fact it's theoretically better as pkg because you can have control over the emulator tweaks, but (just like PS2 software emulation configs) most people don't even remotely have the knowledge to even make an educated guess at which options to try, because they require knowing how the game works in the first place...
It's just very inaccurate (no support for loading separate executables, no support for direct umd sector access, ...) so don't expect miracles even if you knew