Hi Johnnyj,
Perhaps the best place to begin is here The basic functions of a mattress - Overview - The Mattress Underground
The best way to look for a mattress is to focus on the materials rather than the brand. In practical terms … most comfort layers will either be memory foam or latex or polyfoam but it’s important to make sure that the comfort layers in a mattress use good quality materials. The lower quality/density foams used in the comfort layers of most national brands (or any lower quality material)… should be avoided because they will soften and break down more quickly than higher quality materials. Other less common options include comfort layers made of natural fibers, microcoils, or buckling column gel. These less common or more specialized options may be suitable in some cases however in general terms your choice will usually be between memory foam and polyfoam (make sure they are the highest quality available in your budget range), and latex. If you are forced to choose a comfort layer that has more than an inch of lower density polyfoam because of a very low budget … then it should only be from a factory direct manufacturer or specialty sleep shop that will be able to tell you exactly (and truthfully) about the quality of the polyfoam and how long it can be reasonably expected to last.
Latex has many advantages over memory foam however there are some who are used to sleeping on memory foam and like the feeling of sleeping more “in” the mattress than “on” it. In terms of pressure relief they are roughly equal and they are both good at motion isolation although memory foam would have a slight advantage here. In terms of support, breathability, temperature control, health concerns, environmental impact, and durability, latex is generally a better performing material. It’s also however a more expensive material and comes in different types (the raw material used to make the foam can be either natural, synthetic, or blended rubber and is made using two main methods which are dunlop or talalay). More information about the different materials used in comfort layers is here Mattress comfort layers - Overview - The Mattress Underground
The support layers of your mattress may be an innerspring, polyfoam (its not as big an issue in the support layers of a mattress as it would usually be higher quality and firmer when used here), or latex foam. The choices here are usually about how well the material or layer can keep you in alignment, overall feel, budget, and durability as all three materials can be made to provide good support. The best performing and durable material here is also latex. Innersprings and polyfoam can be used for lower budgets or in the case of an innerspring for a different overall feel. More information about support layers can be found here Mattress support cores - Overview - The Mattress Underground
In terms of value … the best choices are usually local independent mattress manufacturers who sell factory direct. Other good options include smaller sleep shops who sell smaller more local brands or higher quality but smaller national brands and are open about the materials inside every mattress they sell (so you can make sure there is little to no low quality polyfoam in the comfort layers). These types of outlets are almost always the most knowledgeable about what you truly need in a mattress (as opposed to what they want to sell you) and more open about what is in the mattresses they sell. The larger chain store outlets in some cases sell mattresses that are useful for testing purposes to give you a good idea of the type of construction that would be best for your unique needs. They rarely have good value however.
If your field testing gives you a clear idea of the type of construction and layering that is best for you but there is little available in your local area to actually buy that has reasonable value … then there are also online outlets that can “duplicate” the type of mattress you prefer and that do have very good value. Even at the best “so called” sale prices … most people end up paying 50% or more than their mattress is really worth, buying a mattress that is not suitable for their needs, or buying a mattress that uses too far too much poor quality materials that will break down far too soon.
The thickness of the layers … especially the comfort layers … is also a very important part of how suitable a mattress is for a particular weight, body shape, and sleeping positions. Side sleepers usually need a comfort layer “in the range” of 3" and this is a good place to start your testing. It can be slightly more or less depending on body weight and shape. Latex comes in a range of softness choices while memory foam would all be considered soft (in actual use at normal sleeping temperatures even though it can be quite firm when it is colder). This comfort layer needs to be thick enough to relieve your pressure points, to “fill in the gaps” in your sleeping profile (lumbar/waist area especially), and to allow your shoulders to sink in deeply enough to be in alignment with your hips (to relieve shoulder pressure and to help keep your spine in alignment).
The support layers need to hold up the heavier parts of your body (primarily the hips) which tend to sink down into the mattress too deeply and put your spine out of alignment which can lead to or aggravate back issues. They are the part of your mattress that primarily keeps you in alignment. In some cases, the different types of layering or materials or design can also “help” the comfort layers with pressure relief. As long as a support layer keeps you in alignment in all your sleeping positions … then the choices between them are more about how they feel, how durable they are, and their overall cost and value.
The durability of the different layers of a mattress is (or at least should be) one of the biggest parts of its cost and overall value.
In your area (General Philadelphia area)… there are a few local or regional factory direct manufacturers that may be well worth visiting. They include …
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Regional factory direct that makes a wide range of mattresses
http://www.croydonmattress.com/directions/index.html Local factory direct that also makes a range of mattresses
ADDED: Post #4 here has a more complete list
Some local outlets that may be useful for testing include
http://www.sleepys.com/ Has a brand called Pure Latex Bliss which has different models and can also include different toppers. These are very useful for mattress testing and the specific layering of each model is known and can be easily duplicated at a better value.
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Also carries Pure Latex Bliss at their showroom in Philadelphia
EDIT: Sleepys no longer carries PLB mattresses
Take your time and don’t let anything or anyone create a false sense of urgency with “fake sales” or other sales techniques. High value and high quality mattresses are available every day of the year from reputable outlets. A few guidelines that can help you “cut through the noise” of all the misleading “stories” you will come across are in reply #6 here.
Hope this helps a bit and certainly feel free to post with any questions along the way 
Phoenix