Cthulhu Rising is a deep space setting in which the PC astronauts encounter creatures of the Mythos, since deep space is where the baddies came from in the first place.
Cthulhu Tech is more of an Earth-based setting of monsters vs. giant robots. Think Toho's Godzilla task force, only they're fighting Mythos critters instead of Japanese kaiju.
If the existing BRP sci-fi material doesn't suit, you can't go wrong with
Traveller. It was "the" science fiction game in the same way
Call of Cthulhu was "the" horror game. Also take a look at
Rocket Rangers!, a free pulp sci-fi setting I wrote for
Mini Six, available for download at
AntiPaladin Games. Compared to
BRP and even
Traveller, Mini Six is pretty quick and dirty, and it uses regular six-sided dice. You can be up and playing in a few minutes. Also consider
Legends of Time and Space, another freebie for download at
www.darkcitygames.com. It is based on the old
The Fantasy Trip, only four pages long, and the free adventure provides basic spaceship combat rules. Another easy to learn and play rule set.