Saturn :: 2004 - CEL Comes On - Replace Catalytic Converter?

The check engine light came on so I took my car in. My mechanic whom I've used for years told me the following: the catalytic converted had started to come off, in the process it pushed the oxygen sensor up against the body of the car and shorted it out. The total damage is that the O2 sensor and the cat has to be replaced. The line-item estimate was $104 for a sensor and $650 for a cat, plus labor. Total: $1000. Now, I'm looking online and seeing cats that work on my car for far less ($200-400 depending). I talked to another mechanic on the phone who verified cats don't have to be $650. My mechanic said they only use top quality parts with warranty, etc. But the other mechanic said that for a car that he wants to get moving again, a cat is a cat and any new part will, of course, come with a warranty. So... what should I do? Is $1000 too much? If I go with the mechanic who says he can get me down to $700, will that be cheap work on my car? It has 108K miles, and I'd like to get at least another few years out of it.