1.5 has the advantage of Devhook. You can either play the UMD version at 222mhz (normal clock speed) or you can "overclock" (the PSP processor was made to play at 333mhz, but lowered to 222mhz for longer battery life) it to 333mhz for much faster loading times (which helps a lot with games like Smackdown Vs. Raw & Untold Legends.) Along with all the other features of 2.71 (rss, location free, internet).
The emulators arent too great on the PSP, quite a few SNES games dont run too well, and the GBA emulation isnt much better.
The only downfall really is that when they come out with a new firmware, were left in the dust (assuming there are any decent updates) for a random amount of time. So far 2.8 has already been dumped, it just needs to be decrypted to work with devhook and then we'll be able to have RSS video as well.