It can make a definite difference, especially if multiple upgrades are purchased. Enemy units designed to be able to absorb two or more hits suddenly start blowing up early. A missile crawler with two shots can suddenly both move and fire. Buying an extra shot for a vehicle is always worthwhile, if you can afford it. The classic tactic is to put your constructor to work on a Light Vehicle plant while the Engineer works on storage units for the mining station and connectors between the plant and the mine.
Extra engineers can set up some fixed defenses, like anti-air, radar, and missile installations.
Early on, you also need a heavy unit plant to build the major fighting vehicles, and an air unit factory. Once you have these facilities working, you're left to decide which units to build in them. Early on, scouts are always useful. Extra surveyors can be good if there is a lot of land to check for vital resources. Engineers and supply trucks will always find work, too.