[danweber]

In space, in the inner solar system, solar panels are more power dense than RTGs.

The situation might change enough being on planet where the sun is slightly filtered by the thin atmosphere, and the sun doesn't shine ~12 hours a day.

RTGs have many other advantages: they provide heat as well as power (which is really important on Mars) and they don't wear out when covered with dust. If Curiosity stops running within a decade, it won't be because of power.