Martha, while often whining, was so much more fun than Donna. Donna just sat there and complained for half of the show. Yeah she got stuff done, but so did Martha, and and Martha didn't have to throw a tantrum every time something happened. Yeah there was an awkward love interest, but if you spent that much time with an amazing person doing amazing things, wouldn't you wonder why they didn't start loving you. Donna was in it for the sights, Martha was in it for the Doctor. Donna wanted what the Doctor had, without really caring for the doctor. Martha cared for the Doctor and his adventures were an added bonus.
After Rose left in Doomsday, the Doctor and Donna defeated a giant space spider. The Doctor asked if Donna wanted to come along, but she declined and I thought that she did this because she did not think that anyone would want her to come along on any trip. So she told the Doctor to find someone. Martha only went with the Doctor because she had a crush on him and thought that she was his equal. Though it is made VERY clear that the Doctor will not love her as Rose she backs off somewhat. Donna never thought of the Doctor as a lover. She was his best friend. Donna could see right through that smile and see a broken and sad man underneath. She understood the Doctor and the Doctor understood her. Both were very affected by Journey's End. The Doctor lost his best friend. And Donna will never know how special she was to him, the universe, Lee.
While neither stacked up to Rose or Amy, Donna was better. Martha frustrated me; she seemed to hold up the show a lot. I'm sure she was written as a more "equal" companion to that of the Doctor, but Donna made the show more entertaining, and I absolutely loved Doctor-Donna.
Martha was great, but Donna was so much better. SHe was sassy and cared about the doctor instead of falling in love with him. She wasn't afraid to stand up to him either. Personally, she was my favorite companion the show has ever had. Amy would probably be my second favorite...
Donna was full of wit and attitude while Martha was the most boring in character of all the companions. Donna always made me laugh, and she made me cry. But Martha just made me mad. She was annoying and stubborn, and always got on my nerves. I don't like the tone of her voice. She is just so full of herself, like she believed herself to be sooo important. But Donna. She wasn't like that. She couldn't even comprehend that she could ever be special.