Screw Mars and Venus, We should be heading to Mercury
* The Craters at the poles are permanently shaded so you don't have the temperature extremes
* Mercury is about ½ the mass of Mars with less of an atmosphere so to blast off the surface would require less fuel.
* It's on average closer to Earth than Mars so you wouldn't have the total distance to and from as you would with Mars.
* Venus can give a boost both going and coming to Mercury(remember Venus boosted Cassini onto Saturn)
* There is Hydrogen and Oxygen in the atmosphere and surface of Mercury so you can make fuel, breathable air or water unlike Mars where you can't make any of those out of Rust.
* There is water ice in the craters near the poles so we wouldn't have to carry as much on the journey
* The Mining opportunities are limitless
* There is Helium in Mercury's atmosphere and in it's rocks (where there is most likely a lot) and unlike Earth's Helium the much of the Helium on Mercury is probably in the form of He³ which is supposedly a target fuel for (cold?)fusion.
The main problem with moving Mercury and Venus out to the temperate zone so they can be properly developed is that astrologers are not going to be able to pretend they even look at the sky anymore.