Rotties and Labs have different personalities--I don't think either one is necessarily a bad choice, but one may be a better choice for YOU. Whichever one you go with, please please PLEASE for the dog's sake, research breeders! Find a responsible, ethical breeder who breeds for the betterment of the breed, who works their dogs, breeds for health and temperament, and tests their breeding stock. Both Labs and Rotties have potentially serious health problems, and due to bad breeding, can have poor temperaments. Labs are the number one dog in popularity in the US right now, and the number one dog ending up in shelters and rescues. In my area, Rotties are number 2.
Both breeds are working breeds--the Lab was bred to spend a day hunting, the Rottie to serve as a guard and drover. They both need exercise (lots) and both need training (lots). Neither one is really a laid-back, easy going, low-maintenance dog content with a walk around the park or neighborhood. You may want to research some kind of dog sport--agility, flyball, tracking for either breed, and for the Rottie, carting or herding. You need to provide mental stimulation as well as physical exercise (BTW, the same is true for your little dogs, too!). If you really want a low-energy large dog, you might consider a Bullmastiff or Great Dane.
I would recommend finding a dog show--a specialty show for Labs and Rotties, or an all-breed dog show--and talk to some breeders and handlers. You can find dates for shows in your area on the Internet. Research breeders; ask questions--what is their goal in breeding? What health tests do they do? Do they guarentee the puppies?
Good luck in your search!