Extra surveyors can be good if there is a lot of area to check for vital resources. Engineers and supply trucks are good. If you get into a fight early on, then bulldozers should be built as soon as possible to take advantage of the debris. Sometimes the debris you pick up off a battlefield can keep a colony alive until the second and third mining stations are built. Which air unit to build is always a good question.
Air units are fragile.
Anti-Air units are very powerful. An AWACs is an excellent investment, especially if protected by a flock of fighters. Upgrading the scan of an AWACs might be very important in the later stages of a game, to avoid being brought down by anti-air units with extended range. As stated earlier, Ground Attack Planes can be devastating. Since airplanes never have to land except to rearm and be repaired in this game, they are best used at the fringes of a conflict, taking out constructors and engineers trying to build new facilities, moving columns of enemy units without fighter support, and wayward surveyors and scouts.