Firstly how old is your lab and is it a male or female. Is the lab the first big dog u have left. Are you looking at breeding and showing or just as a pet.
Rottweilers can be quite different as compared to labs and require more time and energy. Non the less they are a great breed. always see both the parents and only then buy a pup as most popular breeds are being breed and sold very cheap, but that does not assure quality.