I have kept rats as pets for a long time now, and within the past few years I have started taking in rescues. Previously, all of my rats have been from Petsmart (which are wonderful, healthy, and long lived). However, the rescues I have taken in have been cheap feeder rats from Petco, and have cost me hundreds in vet bills.
While all female rats are prone to mammary tumors, lung issues, and skin problems, I have rarely had these issues with high quality Petsmart rats. Every single Petco rat I have ever taken in has had these problems, and the majority I have taken to the vet to be put down after a year or so of life.
I have heard these same issues with small animals besides rats who had been purchased at Petco vs Petsmart. The staff at Petco knows little about how to care for rats, and they are often kept in cramped conditions (twenty or thirty rats in tiny cages kept on pine bedding, rats should never be kept on pine and the general rule is one rat per every two square feet). Whereas Petsmart keeps their rats in excellent conditions.
I am sick and tired of taking in these poor unhealthy animals. As rats, they always have wonderful dispositions, and it is always heartbreaking to get these wonderful animals put down, especially after spending hundreds to get the tumors removed, and just having them come back in different spots.
If you do go the Petco route, be prepared to spend hundreds at the vet. Please consider the Petsmart small animals instead. I know Petsmart isn't perfect, but when it comes to the long term health of their animals, they are wonderful.
(I've been to multiple Petco and Petsmart locations in different states, and I find that this general rule holds true at both locations.)