Personally, I have used ME CFW since the first ME CFW by neur0n since it reminds me of the old M33/Dark_AleX CFW's. I can also switch the IPL in 2 seconds without using the ME/PRO updaters. You can also run a 6.60 HEN and MMS/Time Machine/Pandora alongside ME/PRO CFW so you can actually boot multiple firmwares on the same device without flashing (personally, I can boot any 6.60 CFW/HEN, v1.50, v3.60, v5.00, ect on my slim TA-85v1). You can do more than just dual boot ME/PRO CFW. You can't change it to say 6.61 in System Software as far as I remember without updating to 6.61 but I think with PRO CFW, if you enable version.txt from the recovery it will say something like

Remember, faking your version.txt doesn't automatically allow the running of games requiring a higher firmware (if that was the case, I'd still be on 5.00 M33). Personally, I unpacked the 6.61 Updater and took the one from it to use. You can make your own 6.61 version.txt or just find one on the internet. Version.txt is just something the PSP recognises as the version your PSP is on if I remember correctly (6.60 for example). I've heard 6.61 has a plugin that can load 6.60 plugins by Davee iirc but I've never tried it nor do I know if it works very well. 6.61 hasn't tempted me at all because of plugins, themes and stuff so I stayed on 6.60.

Emulator psp 6.61 mac