Have no experience with stainless but my father bought one of the original Grundy?? ones about 20 years ago which was guaranteed for the life of the car. Had 3-4 replacement sections over the next 10 years, all FOC. One side effect was that because it had less weight in it than the mild steel exhaust it had a different, more aggressive, exhaust note.
On long life the Passat I gave back after 4 years and 96k miles was still on the original (long runs), the A2 which was traded in December at 27 months and 17k (mainly short runs( was still on the original. When we had a car which needed replacement sections I used to buy Bosal Aluminised and on a low milage car (nearly all short runs) they used to last 2 years - could not afford the capital outlay for a stainless one.
Generally OEM original fit last longer than basic aftermarket but OEM aftermarket can be the same as the factors so why pay OEM price. e.g. the Sierra factory fit was one piece from the manifold downpipe but Ford's after market system was two piece and not the same manufacturer as the factory fit!
1.4 Petrol Special Edition, Ebony Black, grey leather, heated seats, climate, opensky, five spoke alloys for SWMBO (Replaced a 2002 1.4 Petrol SE) Honda Accord Tourer 2.2 iCDTi for me.