Matt, what IS that sight?
Oli's right, any big dog has a potential to knock you over. Even little dogs have a potential to trip you. Labs are great dogs and they do make wonderful service animals. If you were really interested in a lab, maybe you could find your local service dog organization and go about training a service dog? My neighbor across the street used to train service dogs and yes, most of them were golden lab pups. The hard part is you do have to give the dog up but knowing that it is going to serve a great purpose is heartwarming and the training would allow you to keep a lab without it being destructive because it's bored and has no purpose.
Dobermans are goofs. Interestingly, speaking of service dogs. I was in the grocery the other day and a woman at the pharmacy had a Dobie with her. The pharmacist said it was her service dog. Although, in some ways I kind of doubted that because to be honest, the woman didn't really look like she needed a service animal. I've seen some people who go and make fake service animal vests for their tiny yorkies or other clearly-not-for-service dogs just so they can take them into stores. It's criminal.
EDIT: DQ, you can still train the dog and take training classes even if it isn't a puppy.