Hi Countrygirl,
One of the most important “messages” of this site as you probably know with the reading you have done here is that no matter where you buy a mattress to always make sure that you know the details of all the layers in your mattress so you can verify the quality and durability of all the layers and identify any weak links in the mattress, especially in the upper comfort layers, which can lead to foam softening and the loss of comfort and support and soft spots under the heavier areas of the body especially like you have experienced. This is particularly true for people who are in a higher weight range where all materials will soften and break down more quickly. Post #1 here and the information it links to will give you the basic information you need to eliminate your worst options and focus on the better ones. It’s probably the single most important post on the site and will help “cut through” the information overload that can sometimes happen in a site like this.
While i can’t tell you which to choose between these two mattresses because the choice between a latex / polyfoam hybrid and a latex / innerspring hybrid would be a matter of personal preference … I may be able to help you with how to choose.
The most important part of your choice would be to decide on the relative “value” of each and on which one best fits your personal “value equation” (see post #46 here).
As you will read in the post I linked … it’s very important to make sure you test specifically for pressure relief and for alignment when you are testing mattresses rather than the more subjective “feel” or “comfort” of a mattress because it will translate into a mattress that is much more suitable in the long term. The final piece of the puzzle no matter how well a mattress provides you with what I call PPP (Pressure relief, Posture and alignment, and your Personal preferences) is to make sure you know the details of every layer or component because if there is a weak link in a mattress it can lead to the loss of the comfort and support that you experienced in the showroom much more quickly. For this mattress … you would need to know the thickness and density of any polyfoam in the mattress (such as the EcoTex extended life foam). The latex is likely to be good quality but I would still want to know the type of latex (Talalay or Dunlop) and whether it was blended or 100% natural (with Talalay latex blended can be more durable particularly in the lower ILD’s or softness levels while with Dunlop it’s the other way around).
I can save you the trouble of finding out the layering of this mattress because I have it. the layering is as follows …
9" height
2" ActiveFUSION Fast Natural Talalay; This is blended Talalay latex with phase change gel added to help with temperature regulation. It is a high quality and durable material.
7" Bonded Foam/Latex: This is a blend of polyfoam and latex similar to carpet padding where pieces of each material are compressed and bonded together. it is also a very durable material.
This would be what is called a latex hybrid which is a combination of latex in the comfort layers and a polyfoam support layer (or in this case a type or latex/polyfoam blend). This mattress is on the lower end of their line because it only has 2" of latex but all the materials are good quality and durable. The latex comfort layer may be a little on the soft side but it’s also thin so that the “feel” of the firmer support layer will come through more.
You can read a bit more about a latex innerspring hybrid (and how it compares to all latex) in post #2 here. You can also read more about a polyfoam / latex hybrid and how it compares to an all latex mattress in post #2 here. Generally a good quality innerspring will be more costly than a polyfoam suupport layer but in the case of the PLB the support layer is higher quality than “regular” polyfoam so the difference may not be as much although how each performs and feels to you would be my primary consideration.
Latex is the most durable of all the foam materials (memory foam, polyfoam, and latex are the three different types of foam) but with your heavier weights it would be important to make sure that any foam layer wasn’t too thick and/or soft or you could risk both alignment and durability. Firmer foams are generally more suitable than softer foams for heavier weights and the PLB toppers are the softest latex available and also among the most costly latex toppers so if I was going to add a latex topper I would probably look elsewhere (to find both firmer latex and better value).
Unfortunately there is probably better latex “value” available in your area than either Novopure or PLB (I’m not sure where you are but this would be true in most areas in Canada) but I also understand that you are locked in to the store you are working with so you will have to make the best choice out of what you have available.which means knowing every layer that is in both of your choices.
Hopefully I’ve covered all your questions and helped with “how” to choose rather than “what” to choose (because only you can know which one is the best match for your needs and preferences). While I don’t have any questions I’m certainly happy to answer any others that you may have 
The next step is to find out the specifics of what is in the Novopure so you can make a more meaningful quality, value, and durability comparison between them.
Phoenix