5e seems to hit that right balance of ease of play for the casuals and enough streamlined strategy for the lifers, so I was looking for something sci-fi with the same happy medium.
I might suggest Alternity, though it is out of print. Copies can be found cheap even on Amazon, though.
It has the benefit of having what may still be the best setting supplement ever written - Dark*Matter. This is for playing something like X-Files level sci-fi. Star*Drive, the full on space opera setting for the game is also excellent.