http://faq.ninja250.org/wiki/Main_Page
Loads and loads of information of the pregen. A lot of new gen info on this forum..
The pre-gen models are the pre-08 models... They're all kinda lumped together. The new-gen models are everything post-07.
Split into the two categories, my personal opinion is that the pre-gen is more practical, faster, and has better ergonomics. The new-gen, however, is sportier, more attractive, and has slightly worse ergonomics. Both are great bikes, and both will serve you well.
Also..there's way too much information out there to just make a thread for it! Search key is your friend..