Catalytic Converter for 2007 Sentra

[revpipes]

I have a 2007 Sentra 2.0 SR. A light came on last week that said, get engine serviced soon." When I had it checked out, the code indicated that the catalytic converter was getting ready to go. One mechanic said that he only deals with Nissan parts because the after market converters often don't work. My questions are two: 1. Is it true that the non-Nissan converters aren't dependable? and, 2. How much should I expect to pay for this repair here in MD?