Actually, despite my above pro-Venus talk, Mars is still an easier option. I wound up on a wiki-spiral just now reading about options for Venus- turns out plants would not work for converting that CO2. The process for CO2 conversion needs Hydrogen- and Venus has none.

We'd need some kind of trade-triangle between Mars, Venus and Jupiter, dumping CO2 from Venus to Mars, then moving H from Jupiter to Venus. We need to find some product Mars has that Jupiter wants, then we'd have it figured out.