Is this similar or big improvement to Battlevoid Harbinger?

I understand what you mean, it's a bit hard to articulate the differences to harbinger while still having the text be good as a general description of the game.

So let me throw some differences of the top of my mind.

One of the main differences to Harbinger is that Harbinger was focused quite a lot on the star map, while Sector Siege is focused on what happens inside a sector. It might take an hour to battle out 1 sector, the star map is much more simple but the sector battle is a lot more fleshed out.

• Max 10 ships + 8 stations to command in Sector Siege versus 3 ships in Harbinger.
  
• Marines added to ships.
  
• Boarding feature added, both the player and the AI can board.
  
• Capture points added, generating resources.
  
• Old and new ships.
  
• Turrets are mostly the same. Area of effect added.
  
• Fog of war added.
  
• Seamless saving and loading added, save and load at any time. Cross-platform save/load support.
  
• Ability to capture all enemy units and stations added, and it's possible to start building them once you have one captured.
  
• 1 new race, the Guardians added.
  
• All alien races may warp in to a fight and attack any other race. This means alien races will be fighting each other too.
  
• Skirmish game mode with 2v2 added, you can play with an alien ally.
  
• Star map is not randomized, but a lot of the elements inside a sector are randomized. Sector Siege is all about what happens inside a sector, and one sector play can easily take 30-60 min. A campaign has 25 sectors.
  
• You can build up the Battlestation.
  
• Defense missions added.
  
• Skirmish also has defense mission and you can play against endless enemy waves trying to reach a high score.
  

These are of the top of my head. If you can piece together some nice sentences I may add them to our description of the game! Of course we will try to evolve the game further based on the feedback after release.