Hi anniesearch,
Welcome to the Mattress Forum! 
I think you may be confused with the nomenclature used in the description about the Plushbed mattress in question. Both the Luma Latex Hybrid and the Plushbeds Luxury Bliss use pocketed springs that are encased in fabric – that is how pocketed springs are constructed. The encasement described is the material surrounding each individual spring, and the spring units for both mattresses are made by Leggett and Platt and bot are fabric-encased.
The Plushbeds Luxury Bliss uses good quality materials. The 3" of latex on top of the spring unit is 100% natural Dunlop. In the “medium” version, the latex is 24-26 ILD, in the “medium firm” it is 29-31 ILD. The organic cotton cover is 1" thick, quilted to Joma wool. They offer a 100 night sleep trial, with a free return/refund if you don’t like the product within that 100 days.
You can see some of my overall comments about Plushbeds in post #2 here . A forum search on Plushbeds (you can just click the link) will bring up much more information and comments/feedback about them as well. They generally use good quality and durable materials in their mattresses and they would certainly be a much “better than average” choice compared to most of the mainstream mattresses that most people end up buying. However, they do a lot of internet advertising so they do attract a lot of online attention with large “discounts”, and there have been comments of some of their sales people being a little too “aggressive” for some people, so I would make some careful “value comparisons” with some of the other options that are available to you.
The Luma Sleep Latex Hybrid Slumber System also uses very high quality materials. Their design is a bit different, using an interchangeable comfort layer topper on top of the base Luma mattress. The base Luma mattress uses a pocketed spring unit, above which is 1.5 of blended Talalay latex. The exchangeable upper layer can be had in 3" of plush, medium firm or firm levels of plushness. The covering used is a flexible natural tencel fabric. They offer free returns within one year of the purchase, with an exchange of the upper comfort layer for free within the first year. Luma Sleep is a member here of the forum, which means that I think highly of them and their products.
Both mattresses you are considering use good quality materials, and while they are using different types of latex and coverings, you’re choosing between “good” and “good”.
If you have any other questions, I’ll do my best to answer them.
Phoenix