sojourner said:
^Of course, to re-spin Venus, you'd have to crash something like Titan, Europa, Ganymede, or Io into it, just as you mention for Mars
There are other ways to impart angular momentum to a planet; mass drivers and asteroid flybys, for example.
Comets or small ice moons would have to be moved to Venus to replace its water, however.
Of course, this is a very long-term project, taking thousands or tens of thousands of years.
Sealed living environments could be constructed more easily on Mars than under the horrendous conditions that exist on the surface of Venus.
Talk about terraforming and "respinning planets" is nonsense - we can't even figure out how to live nondestructively in the environment we evolved in, or even deliberately influence the weather. Science fiction fans love these preposterous notions but in reality we have managed trips of a couple of days to walk around on the surface of the Moon and are not on track to do anything more substantial than that in the foreseeable future. We don't have the first practical clue about how to begin to do any of the things you toss out so casually. We do know a tiny bit about building sealed environments. Even to do this on Mars, of course, would be so costly and complex that it's unlikely to happen for centuries, if ever. And again, none of the people who even try to plan these unlikely expeditions propose going to Venus instead of or ahead of Mars.
The word "terraforming" followed by ludicrous engineering proposals involving moving planetoids out of their orbit is nothing more than hand waving.
Mars is near-term easier to build a yurt on, obviously. So what? No one wants to live in a yurt.
And, sure, there are good arguments to be made that by the time humans can control the environments of either Mars or Venus to the extent where large populations can be maintained there, the need to do so will likely have been rendered moot.
But--assuming for the sake of argument that you want a second Earth--Venus is a far more viable candidate:
1. It is more massive, and has near-Earth surface gravity, which means:
a)humans could live there without microgravity health concerns;
b)it can retain an atmosphere on geological timescales;
2. Venus is closer to Earth. Launch windows are more frequent. A minor point, but worth making.
3. Venus is closer to the Sun, making it more suited for industrial applications relying on free solar energy, or the development of a planetary biosphere, than Mars.
4. Venus has a potential magnetic field. Mars does not.
All that said, of course there's no point in trading one gravity well for another. But if there was going to be a project to seriously transform either Mars or Venus into an Earthlike planet, it seems obvious to choose the one which is already most Earthlike.