Grey - absolutes are for twats.
But if I could only be one or the other - Sith.
There's a lot unsaid about the Jedi in the movies, games and etc. and if you look beyond the protagonist-tinted lenses in which all the movies and games are based, they come off just as if not more sinister, totalitarian, genocidal, and callous than the Sith.
Part of it is due to the series having crummy writers with no sense of subtlety, but if you think about it, it comes to a question of whether you would prefer a system that works by unstable but well-intentioned (usually) extremists, the Sith; or utilitarian space-nazis, the Jedi.
Taking the more thoughtful approach rather than base it off of costume color and whether or not I get to throw force lightning, I would choose the Sith just to have have the freedom to train in the force without some oppressive moral authority dictate everything I do.
It's a very interesting take on light v. dark that sadly gets thrown aside for amateur writers to go "BvZOOOOMM WASSHHKKK CLASSSSHHHHK LIGHTSABERZZZ FIGHT! ZZZZOOMMMM CLASSSSSHHKK VZZZZ!". Where one side is the "Always chaotic evil" side while the other is impossibly good and flawless, denoted by the light and color spectrum of their energy swords.
Of course more serious fans will probably rip a few holes in my previous observations; if so, they are probably right.