A good 'sequal'
While not quite in the same style as the first, and therefore not really a sequal, it's a well made addition to the series. You've really worked hard on this.
Firstly, the gameplay. It's a bit different to the first game, but i'd probably say it's better. I've played through all the missions, and while it can get a little repetative the different maps are quite different. I like that you're forced to use turrets, tanks and stratergy in this one, and not just build a behemoth and cruise the map like you're the deathstar with a blocked up exhaust port. The two layers of units help with this, and add real 'depth'. Pun intended.
Graphics aren't too much of a departure from the first. They're still clean and nicely stylized. Related to this, being able to reflect parts is a nice touch.
Finally, the user uploads. It would be nice to be able to sort the list, because it's huge. A set of weight catagories would be nice, as would some kind of voting system. For every one ship that's been well thought out, with parts chosen for their shape and relevance to the ship's purpose there are hundreds that are just messy, amorphous blobs with parts bolted on anywhere, seemingly taking a 'quantity over quality' approach; as well as many redundant ships, some simple copies of ones already on the list. Over time, I think the noise will drown out the signal.
Overall, a great game. The uploads need a little pruning, but that's not really a problem with the game you made. As a series, it's definately improving, not stagnating. Keep up the good work!