Unfortunately neither the extra layer of latex foam nor the 'stiffer' springs will contribute to a more durable mattress in the long haul. Because neither one of them prevents the low quality polyurethane foams from breaking down in the pillow top. Springs are important, but even cheap springs rarely fail so are rarely a major factor in durability. Out of the two I would probably recommend the Beautyrest as I believe a more conforming spring is best.
But I love how ordinary mattress salespeople are now becoming metallurgists. Personally I would never want titanium added to steel springs, assuming the guage of wire is the same titanium is not even close to as strong as most steels. Nor is it as elastic, which means when it does deform is does not return to its original condition as well; the only upside of titanium is that is a lot less dense than steel so it will be lighter. Carbon however does make steel stronger, albeit also more brittle so there is a limit on how much you would want a spring that by nature is supposed to be flexible. And all steel by definition contains some carbon...chances are high carbon content is also not a very good thing.
In any event, if you have not heard that pillowtops break down quickly you have only read reviews from 'new' customers. They do break down quickly and I would also advice you to stay away from them. 'S Brand' as well as other big mass produced mattress companies like Kingsdown, Spring Air, etc have all taken large drops in quality over time.