When we talk about terraforming a planet the first option that comes up is Mars. There are some good reasons for that as the initial price for creating a kind of space colony is exponentially much less compared to many other options. However, the long term effect of Mars' gravity and lack of magnetic field poses long term problems. Venus, on the other hand, presents a very different option.
As the closest planet to Earth, Venus is considered as the sister planet. It's composition is fairly similar to Earth. Compared to Earth's 9.80 m/s2 surface gravity, Venus' is 8.87 m/s2, while Mars enjoys a mere 3.72 m/s2. This is the most important argument in favor of Venus. Gravity of any planet is one thing we will likely never be able to change within the foreseeable future. No matter how well we terraform Mars its gravity will still not be at a comfortable zone to keep our body healthy in the long run.
Another argument for Venus is that its core composition is thought to be very similar to that of Earth's. That potentially means that the planet can generate a magnetic field of its own to keep life safe on the surface. We just need to find a way to help the planet spin as fast as Earth to imitate Earth days and nights. Currently, one day on Venus takes over 116 days of time from Earth's perspective. Meanwhile, Mars' days are almost identical to that of Earth's at one day plus 37 minutes.
The most important problem with Venus that we would have to tackle first is its atmosphere. It's so dense yet so violent that its winds can alter the rotation speed slightly that manage to create noticeable differences in day lengths on Venus based on measurements at different times. With a mostly carbon dioxide atmosphere, atmospheric pressure on the surface on Venus is over 92 bars and contains thick clouds made up of sulfuric acid. If you can reach the surface you'll be trying to survive at a temperature of over 450 Celsius degrees.
Two major tasks are required to make Venus hospitable; calm the atmosphere and increase the spin speed of the planet. While Mars seems to have a low initial cost but a high long term cost of living, Venus seems to have a high initial cost but immense opportunity for the long term.
Which one would you want to focus on? Mars? Or Venus?