[fallingknife]

Same energy to enter the orbit, but the kinetic energy per unit mass should be a whole lot higher at Venus than Earth, so each of your shots count for a lot more. But yeah, the main advantage is the moon. And you wouldn't launch things from the moon. You would launch pieces of the moon from mass drivers.

The situation I replied to assumed they were both inhabitable planets which I assume means Earth like atmosphere on Venus. The thick atmosphere complicates things, but I don't think you actually have to hit the ground. Tunguska didn't even get near the ground and it still leveled 1000 km^2. Also if Venus has the atmosphere there's no point of a war since there's no benefit to conquest in either direction.