I want to see humans colonize other planets, and Mars is the one that everyone seems to gravitate towards. I know that introducing large quantities of greenhouse gasses could in theory warm the planet, but with only 1% the density of earth's atmosphere, what guarantee do we have that the gasses will stay in the atmosphere? Because Mars lacks a Magnetosphere, it's easy for it to leak gas out to space (and also doesn't protect from harmful solar radiation).
On top of all that, Mars has ~40% of Earth's gravity, which humans are not built to handle. Astronauts aboard the ISS suffer bone density loss that is 10x more virulent than advanced osteoporosis, so even if we managed to settle Mars, we would have shortened lifespans due to the reduces gravity.
Firstly, lets lay down some facts:
Mars...
So lets start with teh atmosphere: - Even thickening without regard for atmospheric content this would reduce the night-day temperature difference (its only a couple/few dozen degrees on Earth, and even smaller on Venus)
Magnetosphere: - Only a problem over very long periods (the solar wind strips gasses relatively slowly), and even less of a problem if you don't want a breathable atmosphere or water cycle (allowing walking outside with oxygen tank/mask and appropriate clothing). The solar wind strongly prefers to strip light ions, using an atmosphere that is only heavier gasses will slow down the effect (This is why Venus still has 90atm [Earth has 1atm] of gasses - which is almost completely relatively heavy CO2 and N2). Again, the solar wind is weaker at this distance than Earth or Venus as well.
Gravity - The weaker gravity is probably the only factor that would increase atmospheric loss somehwat. As for effects on bones, its not so much a problem if you stay there - you wont need as much bone strength in the lighter gravity. Its also worth pointing out that even Mars gravity is much stronger than ISS microgravity or even the Moon. Im not sure how reduced bone density of itself contributes to shorter lifespans (as per your OP)
Water - Mars has alot of water. All frozen and waiting for use to potentially use. Venus has next to none as water splits into light ions and gets stripped from the atmosphere easily.
Day length - having a similar day length is great, especially if you want most people to be active during the warmer day. They'll get a few extra minutes sleep-in compared to us too.
Anyone feel free to correct me on any points.