Venus has little to no water and no magnetic fields. It would never be a good terraforming candidate. Plus it's atmosphere is not just CO2, it has sulphuric acid, too. And I think that Sagan reconsidered his proposal in the '90s as we had better information as to the composition of Venus and it's atmosphere. Mars would be a much better candidate for terraforming, but what we were talking about was seeding a planet to kick start an evolutionary process as a sort of long term science experiment at the very least. I consider the two to be very different things as terraforming would the implicit engineering of a planet in order for it to support earth based life-forms indefinitely.Long ago Sagan suggested seeding Venus with bacteria to break apart its CO2 and help cool the planet. It might be part of a terraforming scheme someday.
Mars has ample frozen water, which might be liquified if the planet can be warmed.