1.5 is the mobi version, and 2.0 is the ePub version, I'm pretty sure.
My personal opinion is that the quality of epub files you can buy or find for free is not good enough to justify abandoning the mobi format. I tried the epub version for a while and then gave it up and came back to mobi. Of course, that means I have to decrypt and convert the epub books I buy, but if I had the epub version I would have the same problem with mobi
Send me an e-mail if you want one or both of these versions. I don't think I can attach it to a PM (or can I? I've never tried it).