Improvements over the 1st game:
-Soundtrack.
-Action/Gameplay.
-Graphics/Textures.
What the 1st game did better:
-Story: the 1st game had a more detailed plot & a comic-style briefing of the mission's objective & story.
-Length/Content: 1st one too varied more with multiple ships, more distinguished levels & overall finality feeling when reaching the final level. Unlike the 2nd game where you're suddenly in a doomsday final boss!