Just so we're clear here, is it the ink (photo vs matte) or the paper (rc vs matte/rag, etc.) that ultimately affects longevity of prints? I'd hate to have these new paper/ink combo prints fade on me after 20 years or so.
I believe it's the paper/ink combination plus exposure to contaminants in the environment and exposure to UV in particular. I've experienced some pretty serious fading of Epson Premium Gloss for dye based inks. The gloss papers seem to be more prone to fading.
I always remember the occasion I bought a stack of Lyson papers and ink for my Epson 1200. A few weeks later, Willhelm published his tests of the Lyson inks and papers revealing that some of them were just as susceptible to fading as the then current Epson papers. The point is, of course, Lyson were marketing their papers on the basis they were more durable than the Epson papers. Unless you can get independent tests of specific ink/paper combinations, you can never be sure of their real longevity.