Technology

Hypothetically, we could terraform Venus. At the very least, it shares a lot of the issues that we're trying to fix on Earth, just dialled up to 11 - its main problems are that it's way too hot, the atmosphere has way too much carbon in it (96.5% vs Earth's 0.04%), and the atmosphere has way too much sulfur (0.015% vs Earth's 0.00000002%, making the atmosphere highly acidic). So if for example scientists had an idea for causing a chain reaction in a planetary atmosphere that rapidly sequestered all atmospheric carbon but were worried about unknown strength or side effects, instead of testing it on Earth where it could kill us all, they could test it on Venus where any failures would have no serious consequences. And if it worked, not only would it mean that we fix climate change on Earth but we partially terraform Venus into the bargain.

Venus has roughly similar gravity to Earth and has a ferrous core which could hypothetically be turned molten (and therefore ferromagnetic) to provide the same kind of magnetosphere that Earth's core does. Mars has neither of these things and would therefore never be able to sustain human life naturally - Venus potentially could. On Mars, the atmosphere is just one of many obstacles. On Venus it's THE obstacle. Solve the atmosphere, you solve Venus.