The problem with Martha was that she was in lurv with the Doctor, but he didn't feel the same way. So that created an annoying dynamic. Again, Rose's influence there. With Donna, there was some of the "Companion saves the Doctor" stuff, but it wasn't like she was somehow "destined" to do that. At least, to the extent they played with any of that stuff, it was fairly limited. Donna just struck me as much more grounded of a companion than any of the others.