Honestly I think that both are vital to survival once we leave the planet. Air can be created in space, as can water. If you look at the first link you will see how breathable air is created on the ISS and the reclamation system in the last link talks about how the ISS gets its water.
In The Expanse(TV), a speculative fiction currently on Syfy, water is mined from the asteroid belt in the form of ice. It claims that water is more precious than air… In a way this is correct. On the ISS air is created by Electrolysis of water. I would be interested to see if any of the international space organizations or one of the commercial space companies can find a way to safely use the jettisoned hydrogen as part of a propulsion system. Also if there was a way to harness the methane that is produced by vegetation and human waste that could also be conceivable if used as a propulsion system.