"No way! Nod's base defenses are ten times better the GDI's! I've takne out the AGT with only a flame tank, its really easy"The Obelisk does more damage yes, but the AGT has a better rate of fire. As for the turrets, they won't be that effective in the final game. The AGT has been tweaked accordingly in the beta, you cannot take it out with just one ftank, unless of course no one is repairing it or shooting at you 
Just watch how the basic stuff work in the demo, that's why they released it in the first place 