It occurs to me that, with the exception of the one Soviet attempt, there have not been any serious proposed landing sites within Hellas.
The atmosphere is thicker at the floor of the basin than elsewhere on the planet, offering both more effective parachute braking (and thus a heavier lander, potentially) and the rare possibility of liquid standing water. There are more clouds and therefore a potentially more active hydrologic cycle. Rainfall on Mars post-dated the basin formation, as there are dry riverbeds observed on both the inner and outer slopes of the basin walls. And while some of Hellas' interior appears to be relatively unaltered lava fill, some good amount of it appears altered, while a small but significant portion of the basin floor appears not to have been covered by lava fill at all.
Is the only negative to a Hellas landing the fact that it's not representative of the rest of the planet? I'd think the interesting positives would outweigh that negative, especially when Mars sports such a wide variety of terrains and surface conditions...
-the other Doug