Clean space is like star trek. Where there are good guys and bad guys and it's obvious. And where the physics are sorta fuzzy (E.i. Warp drive, Phasers, etc.)
Dirty space is like Firefly. Where there are good guys who may do bad things and bad guys that may do good things. And the physics is quite real world. (E.i if you shot through a window you'd be sucked out. No magical warp drive, no lasers, you need fuel to move through space but once your up to speed you can shut your engines off and drift at the same speed without fuel consumption. Basic Newtonian physics.)
Which Space is your favorite Space? For me, I'd have to say dirty. Mostly because made up physics irritates me.