Placing a balloon on Mars has unique challenges. The inflation of a balloon while floating slowly towards the surface requires a relatively large parachute to slow the capsule down and provide extra time to inflate. The inflation and detachment of the balloon from the backshell and parachute could take a minute or even more. To provide that extra time, the balloon will probably have to enter and inflate in an area of relatively low elevation. This provides more time for the inflation.

In addition to elevation, it is important to provide enough solar power. On the equator of Mars, the greatest amount of light will fall on the panels to keep the instruments, camera(s), and radio working. NASA has historically wanted to place landers between 30 degrees North and 30 degrees South of the equator.

Dust is a factor, but not as much for the balloon. A lander setting down in the middle of a very dusty area could starve if the panels are covered by too much dust. Dust also gets into mechanical parts and increases friction. For the balloon, airborne dust is an issue but the airborne dust is omnipresent and is more dependent on seasonal weather than geography.

Finally, the camera(s) should have something interesting to image. Layers terrains, water created features, current water or ice deposits, ancient shorelines, and similar features are all of interest. If the payload can stay aloft for long enough, the instruments should be able to capture a large amount of data as long as the balloon inflates close to areas of interest.

Prevailing winds are a factor as well.

Shown here is a Mars altitude map produced by the MOLA instrument that orbited around Mars:

Areas near the mouth of the Marineris complex have low altitude and interesting features and would be a good target landing site.