Wes in my last post had a comment about choice of systems over others that got me thinking. At the minimum doing a post on "why this system vs. that one" is a good mental exercises....so here goes.
I happen to be very much in to science fiction gaming, but as I have slimmed my collection down I have narrowed focus to Traveller (Mongoose's 2nd edition version) as the main go-to game, and I retain several generic systems that can get the job done, too: specifically Genesys Core, Cypher System, Savage Worlds and GURPS. BRP could technically count, but has never had an adequate all-purpose SF supplement released for it, unfortunately.
Traveller is, to me, very much to SF what D&D is to fantasy: a system which captures the core essence of a defined experience, which (so long as you accept it's core conceits) is the best toolkit and design book for whatever you need in the given genre. It's a little rusty around the edges (as is D&D when you think about it), but Traveller's got a fanbase of excellent designers out there producing good content for it. Just look at the Mindjammer sourcebook for Traveller to see how flexible it can be as a system.
More to the point, if Traveller doesn't quite do it for me, then each of the generic systems I mentioned offer up some robust source material for running your own science fiction settings. If I want fast, furious action-filled sci fi then Savage Worlds' SF Companion is excellent. If I want a really flexible design with lots of story-focused systems built in then I'm finding Genesys Core and Cypher System both offer a robust toolset. If I want hard SF with all the bells and whistles turned to max then GURPS is still king here, even though it's 4E Space sourcebook is getting long in the tooth.
I also have kept White Star around, because it is the only game of its type rigth now, a sort of genre mashup with a old school style that manages to convey an experience both fun and unique. When I want laser swords without any need to explain the physics? White Star gets the job done just fine.
Despite being "SF" to some degree, Starfinder doesn't quite fit this mold to me. Starfinder is a game about playing a D&D/Pathfinder style experience in a space setting with lots of SF and fantasy trappings mixed together. It's not even about "magic in space," it's about the idea of the D&D scenario in all its glory written in a sci fi setting, but with all of the D&D tropes fully extended in to that setting, too. Or Pathfinder tropes, in this case.
Games that I rejected for my collection or gave up on include:
Stars Without Number - I played this (ran it) for a bit and found that I disliked the fact that it simplified the skill system from it's 1st edition, didn't feel that it's design mechanics meshed as well as I wanted, and that it felt overly complicated for a game which was ultimately still "less complicated" than its closest inspiration, Traveller. Ultimately SWN felt too much like a homebrew version of a more well developed/designed game for me to enjoy it. I also was not a fan of its default setting.
Coriolis - beautiful game, but hard to engage with outside of the aesthetics. A problem I've had with other games as well by the same developer, but may say more about my tastes than the quality of design with looked top notch. In the end, I prefer games which give me tools to do my own world building, rather than rigidly defined by their predesigned universes.
There's a range of horror/SF mashup games as well, I gave up Void Core, Shadow of Sol and others in preference for Cold & Dark, which I love for its design and aesthetic. It does have a default setting, but its one I can work with.
In the end, though, it's games which give me more freedom of design with the setting I want that I enjoy the most. Traveller definitely does that, as do the generic systems. The more tightly defined a game is with its world setting, the less useful it was to me.