I would go to an independent muffler shop (not a Midas, Sear, etc.) and have them replace the mufflers with the same ones they would use if the factory mufflers needed to be replaced. That should provide the same amount of back pressure for the engine to maximize its performance and avoid problems with the exhaust valves and other components.
Beyond that it really does not matter so long as there is a free replacement warranty. If I had to pay 25% more to get a better made muffler with thicker gauge materials I would not hesitate to do so.
With my 4 Toyota trucks the factory mufflers lasted well over 100,000 miles and the replacement mufflers lasted until I sold the trucks and were still in good shape. No need to over think this. Just avoid the chain store muffler shops as most are crooks and the rest are not in business any more.
