I put a Cherry bomb muffler on my truck which is a typical knock off of a Flowmaster. The guy at the shop pulled the Flowmaster out of the box and the Cherry Bomb out and they were identical. I put the Cherry on cause it was half the price.
But you are correct with the saying that the Flowmaster will rot quicker. It will because 95% of the Magnaflow kits sold are stainless. My exhaust is not quite 3 years old and I doubt its going to hold together for another full year.
The salt and sand chew threw the aftermarket NON stainless stuff like crazy. For this reason I would definitely go Magnaflow. You can get a stainless steel kit for a super good deal if you look around. I will be going with a Magnaflow stainless catback system for my Ram as soon as I get the extra cash or this one rots off. Should be by next Fall.
Anyways good luck and maybe post some pics or vid once you got it all installed.