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IMO Mars doesn't necessarily seem like it will be the most interesting destination: It seems likely that once we have the technology for extended in-space habitation to get there, the bottom of a large gravity well seems a relatively boring place to hang out. What does the planet get you? Gravity? Spinning habitations seem easy enough. Meteorite protection? We'll need to have figured that out anyway. Land? Is it really easier for farming than in-space?
The asteroid belt looks a lot more tempting to me because even if resources are scarcer (unclear), they are easier to move elsewhere.
Radiation resistance is a big deal.
Of course, the irony is that radiation resistance works against Mars, because what you want is either a) a thick atmosphere (Earth, Venus, Titan*) or b) low-enough gravity that you can go deep underground easily (for which asteroids and even Luna beat Mars handily).
*Not discounting Venus because its CO2 atmosphere permits cloud cities. Discounting the giant planets because their H2 atmospheres don't.
Do they not? Isn't the hydrogen atmosphere of Jupiter rather dense due to how cold it is? I'm not sure what the math would look like on a Hot Hydrogen Balloon. (Edit: like 2-1 density ratio between 100C hydrogen and -100C hydrogen, you'd only have 1/6th the buoyant force of a hydrogen balloon on earth. But double check my napkin math before trusting it for your Jupiter colony please)
As a less-relevant point, I did double-check your maths and I think you did make a mistake somewhere.
Hydrogen at (old) STP (0 C, 1 atm) has a density of 0.08988 g/L. Assuming ideal gas, that's a density of 0.0658 g/L at 100C and 0.1418 g/L at -100C, for a buoyancy of 0.0760 g/L for your hot hydrogen balloon in cold hydrogen at 1 atm.
Air at (old) STP has a density of 1.2922 g/L (representing an average molar mass of slightly under 29, due to contributions from N2 at 28, O2 at 32, Ar at 40 and H2O at 18, whereas H2 is 2). As such, a non-heated hydrogen balloon in 0-degree 1-atm air has a buoyancy of 1.2023 g/L, which is 15.8x the buoyancy of your hot hydrogen balloon (or 14.5x if your "hydrogen balloon on Earth" comparison is at 25 degrees and 1 atm).
I think you might have divided the density ratios of air/hydrogen vs. hot/cold hydrogen, but the relevant criterion for determining how big a balloon you need is the absolute difference of the densities. You need 15.8x as big a balloon to support a given weight with your setup as you would at STP with a hydrogen balloon in air (actually somewhat more, because the lifting gas has to lift the balloon as well as the payload and the skin of a balloon with 15.8x the volume weighs 6.3x as much for a given material/thickness).
(Jupiter's atmosphere does have about 14% He, which makes the numbers a little better than with pure H2, but not much. And yes, that does bring up the possibility of using a pure-hydrogen balloon without heating, but between the buoyancy per litre being even worse than in your example at ~0.0192 g/L and the thick balloon walls needed to keep He and H2 apart in the long-term (they're both notoriously-difficult gases to contain), I think you again wind up in "theoretically possible and could totally let an atmospheric probe float for a few hours, but not practical for long-term holding up a city" land.)
Thanks. I got as far as 0.076, but not sure where I made the math error after that.
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It's theoretically possible, but a) it's still weak (particularly since it's not breathable, whereas a cloud city on Venus counts all the air toward lifting gas), b) it's an active system which kills everyone inside a day if it's turned off, which generally falls under the heading of Bad Ideas.
(On Earth, the slow buoyancy failure of a hot-air balloon usually produces a survivable if bumpy landing. But, of course, that's no help on a giant planet.)
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I actually think there's a good chance the moon does very well for exactly these reasons - there's water ice there, there's enough gravity for useful things but barely enough to stop you from traveling, and we could make a space elevator from conventional materials. Basically has most of the benefits of a space habitat but doesn't require space infrastructure assembly.
One thing that I think planets have that space habitats don't is more room for error. If you are building on Mars it's pretty easy to build e.g. a "panic room" for a colony - food stockpiles, an extra reactor, etc. (And if something does go badly wrong you at least have resources on hand that don't have to be flown to you.) You can build redundancy on a space colony as well, but I imagine it as the difference between designing a ship with that versus a land-based colony. Both are doable, but it's probably going to have a marginal impact on the ship's cost moreso than that of the colony.
This isn't to say that space habitats won't be a thing, though - they seem plausible to me.
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