The 1.5 add on is for some homebrew. You'll still be able to play your backed up games (if you do that), and PS1 games.
The Difference is for us PS3 users mainly. If our main account gets banned, we loose everything we've paid for. Extra tracks for Motor Storm. Add-ons for Soul Calibur 4. Warhawk. If we can't log on as the main account for PS1 games, we can't transfer them to PSP.