Those two sites should give you what you need. There are guides in the forums of both sites that give you the tools and the information to downgrade. It's very straightforward.
The difference between 1.5 and 2.0 is that 1.5 doesn't have a built in web browser, however some of the homebrew guys have developed a web browser so that gets round that problem. There are other subtle differences. The major downside is that the 1.5 version cannot play GTA Liberty Stories, yet. However you can upgrade and downgrade as many times as you wish so again the positives of downgrading far outweigh the negatives. Whatever you do don't upgrade to the latest version 2.5 as it is impossible to downgrade again.
Spend some time on those sites, especially the forum sections, do some reading it'll pay dividends in the end. The money you forked out for the PSP, I think they are quite expensive, will seem worthwhile as it's more than just a toy.