Hi Sleepseek,
Both the Spa Sensations and the Night Therapy are made by the same company Zinus which means they would tend to have similar value (lower price would mean either less material or lower quality materials etc). In terms of comparing them to the Novaform ... they would probably have similar value here as well. A true value comparison can only be made though if the specifics of the foams used in each mattress are known which they don't have listed. For me ... knowing the materials used in a mattress are far more important in determining value than the brand name that is attached to it.
Most "discount" brands sold through big box stores are fairly competitive in value. They are all likely 4 lb memory foams at most ... but this is often not specifically shown on the descriptions and some of them could well be less in which case I would pass them by completely. I personally would never buy memory foam without knowing the density of the foam and if 2 memory foams are the same density and thickness and both manufactured by a known and CertiPur manufacturer, then IMO the comparative value of the can then be determined by price. While this wouldn't take into account the difference in feel between different memory foams or the non memory foam layers, it is the only way to really compare value based on the durability of the foam and its tendency to soften and change over time.
I also believe that Aerus memory foam in the 4 lb version is a great "budget" choice as it is a known foam with good characteristics and is available in many budget mattresses. Overall though ... in terms of a value comparison ... foam density of both the memory foam and the polyfoam underneath it is the most "accurate" means of comparing quality and price (along with knowing the foam is produced by a known manufacturer who is CertiPur certified which both Sleep Innovations (Novaform) and Zinus are).
When you are buying a budget mattress online from a big box store with a great return policy ... you are usually buying a mattress which will have low/mid quality materials that are designed to fit "average" preferences. None of these will have the durability or feel of a higher quality mattress (at least for long) and because every type of memory foam made by different companies has different characteristics (density, temperature sensitivity, response time, breathability, etc.) and is used in different combinations of layering ... it is usually best to just buy the one that seems to be closest to what you need and then trust the return policy if it doesn't work out. The return policy is "traded" for actually being able to know through testing if the mattress will be "perfect" for an individual. Knowing the density of the foams used at least will give a good indication of how long it will take for the foam to begin degrading.
Most of the discount mattress manufacturers at these stores will have similar "value" in terms of actual cost of material ... but each mattress they make may have very different feels. They also may put their "value" in different areas of the mattress. Since the "weak link" in a mattress is usually the comfort layers, then it makes sense to me to put particular emphasis on the quality of the materials used here and in the ticking ... especially with memory foam which will tend to soften and degrade over time anyway (faster with lower quality foams). An important part of value IMO is also knowing how a mattress will feel for each individual and this is not really possible when buying a mattress online, especially with memory foam (unless you've tried the specific mattress you are buying or a mattress with very similar materials and layering). Overall, I would also tend to buy the thinnest and highest quality memory foam layer that was likely to suit my needs (since IMO many memory foam mattresses have too much memory foam in them to cater to the widespread consumer belief that "more is better" and sell more foam).
If I was buying a "discount mattress" online ... I would make sure that the foam specs were known ... especially of the memory foam density (even though the support foam and the layering will also make a big difference in how it feels and performs for each individual) and then just buy the one where the thickness and quality of the memory foam seemed closest to what I needed for my weight and sleeping positions. For those who want to know ahead of time how it will actually feel ... then personally testing a mattress (or mattresses that use similar materials and layering) becomes very important and the only way to know for sure or even "guess" about an online purchase. If you have tested a mattress with a specific layering that is close to what you like and need ... then it is easier to "duplicate" but even here the different formulae used in different memory foams will create a different feel. Polyfoam of the same density and ILD will be much more similar between different foam manufacturers. Memory foam, even in the same density between different manufacturers and or between different formulae made by the same manufacturer will have much wider differences in how they respond and feel.
For an online "budget" purchase I would base my choices on knowing the materials (not the mattress brand), compare the relative quality of the memory foams, the layer thicknesses and construction, the ability to "customize" the mattress after you have bought it (layers that can be re-arranged or exchanged), and the return policy and the costs and effort involved in returning it. In addition to this I would choose a mattress that seemed most similar to a mattress that I liked in field testing. Beyond this ... I would just "pull the trigger" and roll the dice

.
Phoenix