Cats are a beyatch...IMHO, if youre gonna keep the Xterra more than another 5 years, pony up the $$$ and get the OEM cats installed. Keep your Xterra and drive on. It will be better for all the systems in the long run. Looking at about 2-3K installed @ Nissan.
IME, I installed cheaper converters and it led to other thrown codes and engine issues...which led to the eventual sale of my X.
If youre not gonna keep the X that long, get the less expensive aftermarket cats, plan for the sale, and move on.