Questions concerning Psp custom firmware?

Hi! I'm looking to get my psp 1001 model modded and I was wondering which would be the best to play Isos on? What's the difference between 5.00 M33-6 and 6.39 ME-5? What programs would I need for either firmware to play Cso/Isos? What programs could be recommended to me for these firmwares? Thanks for your time! If anyone knows of a better firmware please suggest it. Thanks again!