There’s a heck of a lot more CO2 in Venus’ atmosphere than there is in Mars’.
For those wondering, the gravity, temperature, and air pressure in the cloud tops of Venus are all near Earth-normal. And, breathing air is buoyant.
Mars is a frozen, dry dump. There is really no value in sending live people there. They would take one trot around the lander and want to go home.
But you will need that sulfuric acid for the hydrogen it carries. Carbon from CO2 for structural members, hydrocarbon for plastics and fabric for gas bags, oxygen to breathe. There is thin gaseous silicides, in case you need any silicon.
It will be hard to keep the concentration of CO2 in the breathing air thin enough not to make everyone stupid, with so much of it outside.
But there is a surprising quantity of metal compound gases in the atmosphere.
I've been wondering about the feasibility of self-replicating floating comet factories that make dry ice comets that could be launched to mars. You could even wrap them in some sort of carbon based exterior to prevent them from boiling off too much in transit.
The composition of Mars' atmosphere is less important then getting the pressure right - at 1 atmosphere, humans don't need suits just respirators, and Earth-microbes will be more then happy to normalize things to suit them (they did it here, after all).
For Venus the big problem is water: Venus is dry. The reason it's in run-away greenhouse is because all that CO2 just will not precipitate into carbonate minerals, which to do so needs water. Again: something comets have plenty of.
The difference between the two in suitability though is the tidal lock: Mars has a day-night cycle. Venus has a day-night cycle but it's 116 Earth-days long - half the planet, if you got the atmosphere under control, would be in a freezing night for 3 and half months. You could settle it, but life would only really survive at the polls. While we can process a planetary atmosphere with plausible near-future technology and local resources, there's no known way we could "spin up" Venus (ironically a fully-tidally locked planet would have much more usable surface area as an oculus-world).
Importing enough hydrogen to take up excess released oxygen will be difficult.
Shipping in enough hydrogen from (say) Neptune might be possible in principle using billions of automated, self-reproducing nuclear powered spacecraft.
If its atmosphere could be precipitated and the carbon freed of oxygen and somehow permanently protected from runaway combustion, the planet's low (2.64°) axial tilt and solar proximity might make a polar existence possible. But the overwhelming excess of oxygen would need to be removed or bound up in water. (The carbon might then be safely kept under water. Or, be crystallized out as diamond, which is hard to ignite.)
Quadrillions of aluminum foil balloons full of nitrogen bobbing in the stratosphere might suffice to bring temperature down.
Perhaps surprisingly, the present 3.5% of its atmosphere that is nitrogen is more than Earth's total.
As noted elsewhere, Venus's magnetic field is not much like Earth's, although it stretches almost to Earth's orbit, and might have crossed it in the past. (Such events might account for Venus's baleful reputation in to the oldest myths.)
On earth we have a magnetosphere that protects our atmosphere from erosion from the deluge the sun gives us (hello Auroras!) but Mars doesn't have that. It's size is an issue too plus it's distance and orbit around the sun now. It didn't used to be so barren, it had massive lakes and oceans and possibly life once upon a time. To terraform it now would take a level of engineering that we don't have, but could, if we manage to not nullify ourselves in the next 200 years.
Titan can. But its surface gravity is less than a seventh of Earth's. It seems unlikely people can live on Titan or the Moon for long without fatal loss of skeletal tone. (Mars might be possible, but why bother?)
https://en.wikipedia.org/wiki/Surface_gravity
Anyways, Mars is an outlier with respect to the "about 1G" crowd, but it's almost identical to Mercury. 0.379g vs 0.377g respectively.
Jupiter is a true outlier at 2.528g.
The moon, on the other hand, has many advantages:
1. shallow gravity well
2. only a couple days away
3. interactive radio
Since you'll need a spacesuit anyway on Mars, what's the point over the moon?
Day/night cycles near identical to Earth differing by less than an hour (2 week long day/nights on moon).
A land surface area near identical to Earth's land area, which means room for vast expansion. (moon is a bit more than 2x Russia)
Somewhat more tolerable gravity at 0.38g compared to .17g on the moon.
Vastly more tolerable temperature ranges. A day on the equator in summer can get up to around 70 degrees F, though nights hit -100 F. The large difference owing to no atmosphere. The moon ranges from nearly absolute zero at night, to greater than boiling during the day.
And many more. These are just a handful off the top of my head.
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And some would consider the distance to be a feature, not a bug. It creates a necessity for a large degree of self sustainability which is ultimately a prerequisite to becoming a multiplanetary species. On the Moon, self sustainability not only more challenging, but ultimately not really necessary given it's just a stone's throw away from a resupply. And necessity really is the mother of invention.
Ultimately I also don't even view Mars as a goal (though it may be able to over millennia developed into an amazing place, and I would be the first to sign on for a mission to such), so much as a stepping stone. By the time the first human settlement is established on Mars, we should be looking outward from there as well. Titan being a possible next destination.
If fuel and machinery can be made on the moon, with its lower gravity well, it becomes far more practical to build big ships to go to Mars and elsewhere.
Unfortunately nobody wants to live on Venus, in part because of that much denser atmosphere, so there's not much interest in making oxygen there.
If your goal is to build an exotic resort for billionaires go to Venus. If you're trying to ensure the survival of humanity in the event of a planet-scale catastrophe go to Mars.
Any attempted Mars colony would die out not long after the last shipment, as materials necessary to its continued survival remain unobtainable there.
Furthermore, Starship is anyway wholly inadequate to establish a continued presence on Mars. If it works fully l as well as promised, it might suffice to maintain a Lunar outpost.
Anyone who hopes to establish a sustainable presence off-Earth must look to O'Neill Cans.
Much like the moon missions, the side-effect benefits that it encourages will likely end up making such a mission worthwhile for the tech alone.
I don't understand your criticism of starship though, would you clarify why you think it's not suitable?
All the choices after that become very unpleasant, of questions about who gets to continue living. The supportable number declines inexorably to zero.
For solar power, you need storage for the long night. A heavy weight on a cable could be played out at night to spin a generator, and then wound back up in the daytime.
A nuke power plant, if you would rather, could be as simple as a naked pile hung inside a big fabric tube with a wind turbine at the top. No need for shielding, cooling, or containment.