Small increases in Armor and Hit Points are not very significant and cost accordingly. Go for a second upgrade in these elements. It can make the difference in being destroyed in two shots or taking two hits and getting back to a repair unit to fight again. Speed is relatively cheap upgrade and can make a significant difference, particularly for units that are normally slow. Fast tanks are always frightening, and fast repair units and supply trucks can make the difference between a blitzkrieg and a deliberate advance.
Shots are the most expensive upgrade for a combat vehicle because they can make all the difference.
A missile crawler with two shots can suddenly both move and fire a deadly attack combined with mobility. Buying an extra shot with a vehicle is always worth it, if you can afford it. A M.A.X. Commander who has just landed on a planet has two imperatives. Build a successful colony and defend the colony. Unfortunately, this is the classic guns and butter problem.