There aren't many differences between vehicles, but the ability to move is one that stands out.
A very mobile vehicle, the helicopter can move both horizontally and vertically at will (with a maximum height).
Another mobile vehicle, the tank is land based. It fires shots at a slow, alternating pace. The cannons on the tank will automatically track enemies on a vertical axis so the ability to move the weapons themselves is not available.
The stationary vehicle of the game, you will only be able to rotate around in circles. The benefit of the turret is that you can quickly aim up and down and you don't have to worry about running into objects.
As a turret, you'll be targeted by enemy forces since you're a part of the base they are trying to destroy.