True, but I'm not sure difficulty is as relevant if the motivation is there. You can't terraform the South Pole. Mars reopens the frontier whereas the South Pole is a dead end
Under the sea starts out way more terraformed than Mars can ever be. It has water, gravity, and is warmer. Mineral wealth is right at hand.
The cloud tops of Venus start out way more terraformed than Mars can ever be. It has air pressure, gravity, water, carbon, sulfur, and shirtsleeve temperature.
What motivation does Mars provide that those don't?
The other option is the Moon, which has even less atmosphere and even bigger temperature swings than Mars.
But the point was, if you haven't even proven you can settle a mildly challenging environment, your odds on an actually difficult one don't look good.
Venus, you could only settle by bringing in everything from outside unless and until you can convert the atmosphere in a large way into building materials — Mars at least you only need to convert it into fuel, because you can do at least some of your construction from rocks.
On Venus, plastics, carbon fiber, water, buoyancy gas, and breathing air (I repeat myself) can be made directly from the air and clouds. Plastic and carbon fiber gives you building materials. Robots can gather minerals from the surface and deliver them by balloon to the cloud tops. Insolation provides abundant power, moreso than here; or, a lightweight, unshielded nuke plant may be suspended a mile or more below industrial plant, supported on its own balloon.
Mars will be much more unpleasant than Antarctica. We don't even know whether people can live for long in Mars gravity. We know that long weightlessness is quite harmful.