Hmm...well, the properly built Doberman isn't going to be much larger than a lab with respect to weight. Probably a couple of extra inches in height, but they run between 65 and 85 pounds on average, depending on whether you get a bitch or a dog.
In my experience with both breeds, I think that Dobes tend to be more predictable when it comes to temperament...but I've only met a handful of Rhodies.
Have you looked at the breed standard and the description that the DPCA puts out? If not, here's a link:
DPCA - A Candid Look At Doberman Temperament "Doberman Pinscer Club of America Illustrated Standard"