Using Hohmann transfer, I believe the energy cost is the same both ways. Of course as you point out we have the Moon, "they" have none. So we'd have plenty of rocks to throw.
That said, we'd have to throw much bigger rocks to penetrate their atmosphere. And the likely (to me) actual plans would be:
Us: launch to the Moon, set up there, launch rocks from the Moon to Venus.
Venusians: launch and travel to the asteroid belt, launch an asteroid toward Earth.
The more I think about it, the more I realize that our plan would be the same as theirs: we'd both be heading for the asteroid belt, because nothing we could reasonably launch from the Moon would put a dent in Venus with that atmosphere.
And if we assume they actually can launch through that atmosphere, we're screwed: if they can do that, they're way ahead of us.
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.
That said, we'd have to throw much bigger rocks to penetrate their atmosphere. And the likely (to me) actual plans would be:
Us: launch to the Moon, set up there, launch rocks from the Moon to Venus.
Venusians: launch and travel to the asteroid belt, launch an asteroid toward Earth.
The more I think about it, the more I realize that our plan would be the same as theirs: we'd both be heading for the asteroid belt, because nothing we could reasonably launch from the Moon would put a dent in Venus with that atmosphere.
And if we assume they actually can launch through that atmosphere, we're screwed: if they can do that, they're way ahead of us.