Stalker, yer thinking RTS style thing. Every type of vehicle has different pros and cons like: a 5t truck can carry a lot of people and equipment. M113 is slow, and doesnt carry as much. Chinook is a totally different category, isnt really maneouverable, and cant land in every place. In X-com (it's suppose to be a clone, am i right?) there were no upgrades, only research. You could research new weapons, better armour, etc. And even then, a new upgrade didnt mean the best, there were different reasons both from tactical and economical point of view to keep the level of technology at a certain level.
And why not to make the mision a bit RTSsish Maybe not uprgades but reserches and ability to buy 5t Truck, M133 etc