Will putting aftermarket Catalytic Converters on my pathfinder be problematic?

What is your question?

I purchased a used 06 Nissan Pathfinder from the dealer, drove it home and the check engine light came on. Went back to the Dealership, who determined that the catalytic converter (2) and the o2 sensors needed to be replaced. They are not under warranty and drove the pathfinder about 20 miles. The dealership agreed to replace them, but with aftermarket parts. Will this be problematic?? Spoke with two other Nissan Dealership service departments and they did not recommend. Stated that the intake threshold pressures would not be the same and I would most likely have to replace them again in a couple of years???? Any thoughts?