Using and up and down method will make your anti-air tougher where there are more and tougher aircraft flying around. I prefer a left/right zig/zag to cluster up a full three rows of anti-air weapons in the middle.
The game itself is basically identical to Robo Defense, but with better graphics, and it's a little less variable and challenging than Armored II: Tower Defense, which has more weapons, more varied target types, and comparable graphics. I recommend checking out both of those for comparison.