Venus, poetically referred to as our sister planet, is much more tempting as a target of terraforming. It has nearly the same size as Earth (98% of earth's size, compared with Mars's ~60%), is closer to the sun, which would both improve plant growth and allow for greater use of solar power, and due to gravitational aspects too complex to discuss here, would be an ideal launching area for missions to the asteroid belt.
Unfortunately, the downsides are much bigger. For starters, Venus's surface is 482C, hot enough to melt lead. The atmosphere at the surface is 94 times earth's pressure, which is so thick that it starts acting like a liquid. So anything we send now would get crushed and melt down. At the same time. On top of that, Venus has little hill-sphere, so using satellites would be much much harder, and Venus rotates extremely slowly, and backwards. (The sun would rise in the west on Venus.) The Venerial (That's 'related to Venus,' btw. It does not refer to sex.) day is only two weeks shorter than it's year. So any plants grown on the surface would die from lack of light during the night, assuming they didn't crush or catch fire. Or melt. Lastly, it rains on Venus right now, but not exactly water. It rains sulphuric acid, a compound that works great in a car battery, but would really suck if it fell on your face, or your crops.
Thankfully, the late Carl Sagan did think up a way where Venus could be lived on today. See, the upper atmosphere of Venus has an earth like temperature and air pressure, and if you were to build a city inside a glass (or other transparent surface, plexiglass would do the trick) bubble, and fill that bubble with earth's atmosphere, the bubble would float like a helium balloon on earth, up five miles. There it would float, and with some of NASA's new ion jets attached, it could be sped into a 24 hour "day" of circling the planet. While all of this is workable, and I would strongly suggest this to displaced people in need of a hard to invade nation (Tibet and Palestine, for instance), it's just too sensible for this blog. Besides, the bubble would only make one city that would need special effort to get in or out of, I want a planet that works like earth. Thankfully, Sagan's city-balloon is the first step in what previously would have been impossible.
The other half of this plan comes from New Mexico, where a group of scientists made a little device, the size of a beer keg, that, when heated to 2600F (1444C), strips oxygen out of carbon dioxide, leaving carbon monoxide that extracts into a small container, to be piped away as fuel. Cooled back down to 2000F (1111C), it releases the oxygen into the atmosphere. If water is added, it produces hydrogen gas instead, with a similar oxygen-extracting process. These scientists are suggesting heating it with solar panels, and keeping it near a coal fired power plant, where it would extract 45 pounds of CO2 from the air per day, pressing it into 2.5 gallons of monoxide to be made into fuel, and then releasing its stored oxygen during the night, resetting itself for another day's extraction. Amatures, I say! I have a much grander, insaner plan!
I plan to have a Sagan bubble-city with a number of these oxygen-cans on movable arms. The bubble city would use ion jets to stay in perpetual daylight. Cans would be raised hourly, with a new can lowered. Only a little solar power would be needed to heat it to the insane temperatures needed, as Venus is already very hot. The cans might occasionally be damaged by the sulpheric acid, but they should extract the hydrogen from the water before suffering significant damage. A human could live in the bubble city, or a city of humans. They would extract some monoxide for their use as fuel, but put most of it onto an off world rocket. For best results, this rocket should go to Mars, which needs greenhouse gases. And Martian people will need fuel too, of course.
Over time, thousands of years probably, these combined actions would use up a large amount of the Venerial atmosphere. The bubble city would find itself lower and lower until it scraped bottom, with no increase in temperature. The remaining carbon dioxide could be used to synthesize baking soda to neutralize the sulphuric acid, as one promising American company has done with their smokestacks. The planet's spin speed could be increased by meteor impact. At this point, plants would be planted, water used to form oceans, and the planet of Venus would be a beautiful place to live.
The faster plan would be to put a solar shade in front of it, with an ion jet to hold it in place against the solar wind. After 1000 years of cooling, it would only be a matter of sawing up, packing up, and rocketing away the dry ice, hitting the meteor, removing the shade, and doing the plant thing. Still, that has the disadvantage of no one being able to USE venus while in the process of terraforming, while still being rather expensive.