On the other hand, I think my players will strain to believe in a Mars that is not colder than Earth: perhaps the icecaps are small because there is little water? Also, I wonder whether it would reassure them to acknowledge the fact that Mars really has enormous differences of relief, and the tallest mountains (Olympus Mons and the Tharsis volcanoes) in the solar system.
Percival Lowell devoted quite a few pages of Mars as the Abode of Life
(1908) to the question of surface temperatures. His argument is three-fold. First, he asserts that Mars' visible albedo (0.27) is lower than Earth's (not known at the time; he estimated 0.75). This phenomenon would extend beyond the visible spectrum, so his final figures for incident energy retained are 0.41 for Earth and 0.60 for Mars.
His second point (which he treats as distinct from the first) is that the Earth is 50% cloud-covered on any given day, whereas Mars is 99% cloud-free. Thus, he says, more of the incident radiation reaches the surface of Mars. From these, he calculates that the average surface temperature on Mars is 48 degrees F, versus 60 degrees F for Earth.
Lowell's third point rests on the length of the Martian year. He suggests that, since the Martian year is twice as long, the extremes of temperatures brought on by the seasons would be even greater than on Earth. Summer temperatures would rise into the 80's (F) and stay there for periods longer than the growing season on Earth. Winter temperatures would be correspondingly harsh, but he imagines that most life would go dormant and ride out the long winter in hibernation.
He then gives a long discussion of mountain biogeography, and infers that a high, flat plateau would be much warmer than an isolated mountain peak of the same altitude. From this he concludes that Mars' flat surface would be much more hospitable than mountain peaks on Earth that exhibit similar air pressures, and so the lack of life on those mountain peaks should not be taken as indicative.
As far as terrain relief is concerned, the dominant view of mountain formation at the time was that they were the result of the planet shrinking as it cooled with age. It's possible that Mars was initially not much larger than its present size, and so did not experience much shrinking. On the other hand, if there were shrinking and mountain-building, the lower gravity should result in proportionally higher, steeper mountains. You can have it either way.