I've taken five different hamsters over the course of three years, and these people gave top-notch treatment. I do KNOW rat owners who are very happy with the treatment their pets have received there. PLUS - these vets are brilliant. Sometimes they're a little too, well, enthusiastic. You have to use your common sense when consulting with them about treatment options.
Rats are bigger.
Tumor removal is less perilous. I think it's a place where vets get more experience between graduation and opening their own practice. St. Marks insistence on payment in full annoys me since I know from experience that other NYC private vets sometimes offer flexible payment plans. St. Marks is probably less expensive, overall, than Animal Medical Center, and much less bureaucratic. The individual vets I've seen have taken liberal amounts of time examining my animals and explaining things to me.