There's a reason why Venus is often called the hell planet. Its surface temperature is 864 degrees Fahrenheit, which is hot enough to melt lead. Its air is a suffocating stew of sulfuric acid and carbon dioxide which creates a runaway greenhouse gas problem, and its atmosphere is 90 times thicker than Earth’s, which would literally crush you. Even with that, Venus is still a potential candidate for terraforming, which is a process that turns a hospitable planet into one that could support life.
As we continue to over saturate resources on Earth, people like Elon Musk, believe we need to start looking to other planets to call home. And that begs the question, which planet is better to terraform Venus or Mars? To put Venus in perspective, it’s a terrestrial planet with similar size, mass and gravity to Earth. It’s also pretty close, we could get there in 5 months versus the 9 months it’d take to reach Mars. And while Venus’s atmosphere is a hellscape, it does give us something to work with. At 50 km above the surface, Venus's atmosphere is quite Earth-like, with atmospheric pressure similar to ours.
So one idea is to build floating colonies that would coast in the clouds. Known as HAVOC - High Altitude Venus Operational Concept - NASA scientists proposed sending astronauts inside an "aeroshell" that would enter Venus's atmosphere at 4,500 mph. The shell would fall away to reveal a folded airship, which is just like a floating blimp. The blimps would then use Venus's carbon dioxide atmosphere to float. Another idea suggests cooling the planet down with a giant sun blocker. A sunshade mirror would be placed in between the Sun and Venus, cooling the atmosphere down and blocking the planet from solar wind.
Terraforming Venus is a complicated task, so how does Mars stack up? For starters, the red planet is super cold, at minus 80 degrees Fahrenheit, and a very thin, unbreathable atmosphere comprised of 95% carbon dioxide. Yet, it's size, structure, and presence of water is similar to Earth. To retrofit this planet, one method proposes importing huge amounts of ammonia or hydrocarbons to create a livable atmosphere.
Another idea proposed by Elon Musk is to melt Mars' polar ice caps by dropping thermonuclear bombs on them. Carbon dioxide would be released and that would thicken the Martian atmosphere. At that point, it might be possible for liquid water to flow on the surface which could start a greenhouse effect on Mars. Ultimately, these are all pretty far-fetched plans and it’ll probably take thousands of years and incredible resources to terraform these two planets.