What is the biggest difference between Mars' northern and southern hemispheres?
The temperatures and aesthetic.
The northern hemisphere of Mars is a flatland that is a normal temperature for Mars, where as the southern hemisphere is a volcanic highland. The north is cold and the south is extremely hot, or at least it was. Now the north is just flat, and the south is old inactive volcanoes.
Here you can see what Mars looks like. And this next image shows the temperature in the two areas.