Solar is definitely attractive - with the asteroid belt at triple the distance from the earth you'd expect n^2 reduction in efficiency ~10x, but without atmosphere you might end up about the efficiency of cells at sea level anyway. A solar sail kind of rig could be just the ticket.
In space, solar outperforms RTG on an energy-per-mass ratio until you get out past at least Saturn.
I love nuclear but it's not always the best answer. (OTOH, it is the best answer for anything that's going to land some place and need to function for more than a few months.)
