Which is more important? CPU Clock speed? or Core count?

[LunaMoo]

Both are important depending what you want to do, typically if that's your bottleneck higher clocks would be more important, but take note that typical bottleneck of PSP emulation is not CPU, but rather GPU, Most PSP games would be fine with 2gh dual core so upgrading just the CPU might not give you much of a desired difference althrough results will certainly vary since each game differs.
Also you didn't mention what hardware you currently have, neither what are exactly those cpu's you're looking at, clocks means little if architecture is different.

Just to mention there are some power hungry features in PPSSPP like texture scaling and software rendering which would require both - high clocks AND core count to fully enjoy, but neither of those are really needed for psp emulation, software rendering is required by like one game which is bad port from older generation and not a real psp title, some homebrew needs it as well, but those are pretty bad and texture scaling is hated by many people since it's a matter of taste and doesn't fit most games.;]

Overall PSP cpu emulation is very light in PPSSPP comparing to many other emulators and as mentioned primary bottleneck would be from GPU or terrible graphics drivers. On top of being primary bottleneck GPU upgrade also scales better since it means seeing nicer and more accurate graphics ~ since old gpu's can't emulate some psp effects properly.