In my opinion, both animals are suitable for a 12 year old but he or she has to make the decision on which one to get. They are both good pets because they are easy to maintain and their feeding habits are almost the same. Hamsters are easy to breed and they are useful in getting rid of house rodents. Guinea pigs are often associated with social grooming and they are normally locked in cages. Check out http://en.wikipedia.org/wiki/Hamster and http://en.wikipedia.org/wiki/Guinea_pig for more information.