Biggest thing I've seen, Sith are all about pure power, however it's achieved. They may lower themselves to help the Empire (in fact, A republic officer actually black mailed me into helping his son).
The Sith are all about their own power and actually almost follow along the lines of natural selection, hence the brutal trials.
Jedi on the other hand, are all about themselves as a whole. I always laugh because Jedi's say don't hate but they sure don't like the Sith. The sith on the other hand as individuals don't care about the Jedi, they just kill who's in their way.