The hardness of a single source of light depends on the (angular) size of the light source, and, as the subtended angle of the moon (as observed from the earth ) is similar to that of the sun, (the sun and moon look the same size), the hardness should be similar.
That's what I was thinking, along with the idea that being far less brilliant a source, there's likely to be less natural fill floating about either, perhaps rendering an even more contrasty image. But I just don't know - never even thought of shooting like that.
On the other hand, the old moon does tear across the heavens pretty rapidly, so long exposures may actually povide a sort of fill from the same light source as it shifts around static objects and illuminates from varying angles - a bit like the moving hand-lamp technique we used to use to paint with light...
Rob C