GC2 is a much bigger game than GC1.
The ship builder is new and lets you design the stats and appearance of your ships. It has all 3D graphics. You can play as the aliens and they each have unique abilities. Ships in fleets fight together. Planetary development and the tech tree are completely different. The AI is better.
I think it's overall a better game, and you should at least try the demo if you liked GC1.