Hi The Riddler,
Unfortunately, there isn’t an overall durability algorithm that can predict the longevity of a mattress based upon componentry used, as there are entirely too many variables involved besides the density of the foam layers.
In the two mattresses you’re comparing, I don’t know the exact configuration of each item with complete details of all padding layers within each mattress. The Regency seems to use a fiber pad along with two different layers of polyfoam, and their spring unit is an LFK with a border rod. The Spring Dreams uses a pocketed spring unit without a border rod and I believe has a layer of foam in the quilt and a layer of polyfoam on top of the spring unit. Both mattresses are two sided, as you mentioned. I’m unaware of the ILD of the polyfoams used, as this would also have an impact upon durability. I’d probably put both items in the same range of durability, as they are both in a more basic price range and attempt to represent a good value for a lower price point. The Regency would probably have a bit more of a “firmish” overall feel with the use of a border rod and the type of spring (LFK versus pocketed), with the Spring Dreams probably contouring a bit more. Between the two I’d choose mostly based upon the comfort preference you desire, pricing, and applicability for your specific needs (all parts of your personal value equation) to determine what might make the best value for you.
Phoenix