>>14747967GURPS or Traveller. Hell, you can even have both, and play GURPS: Traveller. GURPS, especially, has plenty of options for almost any aspect of gameplay, you can run it very detailed, or particularly lite (using Bang! skills, like Guns! and Sword!), play it Cinematically or Realistic, and have as much detail as you and your players are keen for.
Stay away from d20: you'll become a retard who believes that "GURPS is shit, 'cause math is hard" (not knowing from experience of course, only what you'll think you know). And the system is really quite shit. Like, really bad. d20's great for DnD, even Spycraft and a few others (FantasyCraft, microlite d20, etc.), but for a decent sci-fi game, it's pretty crap.