If you were looking at actually colonizing the system a cloud city on Venus (look up the idea) would be easier to make workable because 1) Venus is usually closer to earth than Mars and 2) it's gravity is about 80% of earth's. Now actually landing (much less living) on the surface is a different story. In that case forget about venus. Mars and it's moons would be the only option for doing anything on the surface, such as mining. As for the real reason for going to those places your guess is as good as mine.

My guess for that, particularly with the moon, is in addition to mining resources, would be to build spacecraft and other space structure there. The reason you would want to build it there is because you want it to leave the ground. The fact of the matter is that earth has a strong gravitational pull which is working against you when you try to launch anything. If you launched from the moon, not only would it be much easier to get off the ground, but you could have earth's gravity working for you. Want to go into a low orbit; Earth's gravity will pull you in. Want to leave; you can let your spacecraft build up speed coming closer to do a gravitational slingshot maneuver.