It's all a matter of what you want. Personally I do like both of the breeds, but after lots of research I decided to go with a Doberman (two months away from getting!) :D
Both breeds will obviously have to be properly, and well, trained since they both have the potential to be threats. The Rottie is usually around 110-130lbs whereas the Dobie is between 65-90lbs (approximates) so that may come into play if you're a slighter build person.
Also with the Doberman, they seem to differ depending on which lines they come from. The "American" ones I've seen appear to be thinner, less muscled and obviously a bit lighter... whereas the European ones are thicker, more muscled and obviously heavier.
According to a few different sites I've checked out the Rottie is ranked at 7th place for canine intelligence whereas the Dobie is ranked at number 5.
It all comes down to personal preference of which you think you can handle, which looks better, and which animals needs you're able to fulfill better. With a dog like a Doberman and Rottweiler: do not jump in head-first! Do as much reasearch as humanly possible so you can have a good canine citizen - not one that will help lead to more breed bans.