Good point: I seriously wonder if they considered a dirigible design. Buoyancy + directional propulsion does seem simpler in my space-novice opinion compared to a quadcopter. A blimp would be slower than a quadcopter, but nobody's going to be piloting the thing in real time, so that shouldn't matter.

Yes, that was a mission proposal years ago, a blimp and quads. It was not selected then or rehashed this time. In addition, a blimp only concept has been floated (ha!) as well.

Blimp only wouldn't be able to take ground samples easily