it was just very recently discovered that mars has, in some places, liquid flowing water.
http://www.nasa.gov/press-release/nasa-confirms-evidence-that-liquid-water-flows-on-today-s-mars

the problem with low gravity is that there will be significant loss in bone density and muscle atrophy. the rest you can read here:
http://www.philforhumanity.com/Colony_on_the_Moon.html

asteroids are best suited for mining. To be a viable alternative to Earth it needs to at least be half the size of earth because we will quickly want to scale population.