[azernik]

On the other hand, Mars requires you to live in a radiation-shielded pressure vessel.

I think the real killer is that it's hard to build and maintain infrastructure when floating on balloons - much easier to run a spaceport on Mars, or to get to all the useful surface resource deposits. On Venus you'd have to build literal castles in the air, using only only what you can pull out of the atmosphere or ship in from orbit.

Not to mention Mars is at the bottom of a much shallower gravity well.