Bombard both Mars and the moon with asteroids to begin building up planetary mass. And especially comets because they carry volatiles, compounds with some hydrogen content. Oxygen is not in short supply anywhere, even on the moon. Why waste perfectly good asteroids? The belt is where the action is likely to be in the coming generations. Mineral wealth, low gravity, less fuel required in landing and takeoffs.
Save out 10,000 of the richest asteroids for mining and crash the rest onto Mars and the moon. There's a lot more material in the solar system than civilization can use, so let's make up a couple more habitable planets before we take off for neighboring star systems.
Concerning the water ice on Mars: NASA ought to reconsider why it wants to send men to Mars. What are we going to do when we get there?