The main diffrence between Kazaa and Kazaa Lite is the spyware. I got rid of Kazaa several weeks ago, after leaving a notebook downloading stuff unattended for a almost a week. When I returned, there were 4 applications installed, 2 Internet Explorer Add-ons and a bunch of other crap. Kazaa SUCKS BIG TIME!
I've been using Kazaa Lite ever since, and I have to say it's working perfectly. No spyware, no suspicious apps and the GUI is practically the same. It's the same network, so you'll find the same users and files to download. There are some additional features, like the "SpeedUp" thingie. Also, you alway have 1000 rating, so you can download lots of files at the same time (wich is great).
Don't bother with Kazaa, go with Lite!