Nothing really compares in the "spaceship fleet combat" department.
Starsector combat has the deepest and most smooth experience out of all of them. (if played on even footing as the AI)
Other ACTION space games you can try:
The Ur-Quan masters - The game is old so if you don't mind low LOW REALLY LOW res graphics this is a great game. The combat can get pretty deep and twichy, and story is great and the aliens are always interesting.
Void Destroyer - Tho the game is a TPS style spaceship game mixed with a bit of RTS elements, the combat is really in depth so is the learning curve, but in the end is worth it, the thing that keeps it from being better than starsector is that it has no "free play" mode and the new "sand box" update is just that, a sandbox, not anything fleshed out like the current StarSector campaign. Also the game is ugly.
Fractured Space - This is basically Giant spaceship DOTA. Its good, Visually it is the best imo. Tho player base is some what low finding games with even skilled teams is impossible.
Strike Suit Zero - and its expansion are great non indepth pretty fast paced mission based space shooter, its pretty, almost too pretty. Action is good and fluid, story and immersion is delivered well too. (no *** 3rd rate voice actors and music artists here)