I cannot speak to the Marlin or Remington, nor to your question about coating vs integralization - but I can with regard to durability, with qualifications.
The qualifications have to do with the only two camo stocks I have experience with - an issue T/C Omega/Realtree, and a black synthetic Remington Seven stock that I had camo dipped by Bell & Carlson.
I've owned & hunted with the Omega since "new" for the last 8 years, w/o any special care, and neither the stock nor the SS metal finish has yet to show any wear/abrasions/cracks/etc.
IOW, it still looks and acts as new.
I bought the 7mm-08 Model Seven, "used", about 6 years ago, had B&C dip the stock as soon as I got it, and shot/hunted it for two years w/o any issues whatsoever, before selling it for other interests.
I wouldn't have any concerns with the modern process', since they have long gone through their "testing" stages, and are stone reliable - backed by a factory warranty.
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