Here's a brief summary of the hype about microchips and cancer...
College of Veterinary Medicine
I might do both, but Tucker is already microchipped. A lot of tattoos are placed on the inner thigh where there isn't much hair. And as another DT member pointed out, how many people will be rolling a strange Doberman over to check for a tattoo? One is a visible sign for those who check...the other is potentially more permanent. I've seen some very illegible tattoos, and some poorly placed microchips. If a dog is stolen, it probably doesn't matter how it's identified. If a dog is lost, however, most shelters have universal scanners to check for chips, and know to look for tattoos. I think both methods of permanent ID are viable, and have their pros and cons.