The classic version was always my favorite but like many things it's use had become limited through lack of interest. The new PHP software is very nice and less server intensive and uses a different database making it slightly faster and a little more efficient.
With that said, and I believe the younger generation will argue the fact, you can update the classic software to do everything that today's UBB can do. I know if I really felt like it I could pull it off. The reason I don't try is no one would use it.