Honestly, neither of those options are going to feed your family unless you have a LOT of them. The meat to carcass ratio is pretty terrible. The only thing rats have on guinea pigs is they produce more offspring and grow faster.
If you are looking for more self sustainable meat, I would go with rabbits. A few pairs would produce plenty for your family to eat. They do well on copious grass hay with supplemented veggies. I recommend pellets during pre-apocalyptic times, but feeding them on non-commercialized food is certainly doable.
They would produce a good amount of meat with little to spare in case you did not have access to refrigeration. Killing and gutting them is also easier (dunno if you've ever had to cut up a rat or guinea pig, but it is not easy!)
Rabbits can give birth in 30 days and the offspring are usually big enough in a few months (I'm talking larger breeds here) They are also easier to get and do well in the winter. Guinea pigs can not be outside in the winter. Additionally, rabbit fur is quite useful, making the entire animal useful for some purpose.
As for rats, is be careful giving any animal raw meat. At the vet hospital, a lot of animals (dogs, cats, etc) are brought in for salmonella from raw meat. Not only that, any raw meat that touches the animal can potentially spread to humans from contact or through feces.