Attack adds to the damage done by a combat vehicle's weapon. 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. This can be very discomfiting. And, of course, it is absolutely necessary if the enemy has invested in armor upgrades.
Range is a costly upgrade.
It is also vital. Having tanks with a range and scan of 5 when everyone else has 4 will make all the difference. 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.