Hi theatermax,
You certainly have a challenge with your different body sizes which normally “translates” into different types of an optimal sleeping surface and mattress construction. Heavier weights will normally use firmer foams for the same level of pressure relief and “softness” than lighter weights and will also use firmer foams in the support layers to “hold up” the heavier parts of the body and keep the spine in alignment.
There are two pieces of good news in this … first is that many local manufacturers (and even a few national manufacturers) offer side by side “split” constructions where each side of a mattress can be made and layered differently within the same overall height. This can be a great compromise although if the differences are larger then there will be a middle area where one side switches into the other and the differences can be felt. The other piece of good news is that it seems your GF has a less curvy body shape which means that firmer layers of foam (closer to your needs) may work better for her than a body profile which needed a deeper pressure relieving cradle in the surface of the mattress.
There are three basic types of foams that are used in a mattress although there are many variations within each category. Each category shares a similar feel and qualities within a broader range of characteristics. The first thing I would do is to get a general sense of the overall feel of each category.
Memory foam is a conforming foam which is more “time delayed” in its reactions. It is temperature sensitive and as it heats up from body heat it “flows” or moves away to some degree from pressure which takes more time than a more elastic foam which compresses under pressure. It is partly elastic and partly viscous (thus it’s name visco-elastc) This means that with faster movements it can feel much firmer while with slower movements it feels softer (like slapping water vs slowly sinking in to water). This slower reaction can lead to feelings that some have described like “sleeping in sand” where it is more difficult to move. Others like the stability of the foam which tends to “hold them in position”. It also forms itself around a body shape more exactly along the sides of the body than more elastic foams (think of a valley with sloped walls as opposed to a canyon with steeper walls). Finally it is also less breathable as a category (restricts airflow more) than other foams. All of these characteristics can vary with different memory foams. Because of its lower general breathability and because it forms itself closer to the sides of the body … it is more insulating and can lead to sleeping hot for certain people. It has a feel of sleeping more “in” the mattress than other types of foam which have a feeling more of sleeping “on” a mattress. All memory foams would be considered to be “soft” although there is a variance in how long the softness takes to appear because of it’s sensitivity to heat (colder is firmer) and because it flows away from pressure to a certain degree. This softness means that it is only used in the upper layers of a mattress because it is not supportive enough to be used in the lower layers (like honey the heavier parts will continue to sink deeper to some degree over time). There are actually no full memory foam beds because all of them need more supportive foam or innersprings underneath the memory foam. The quality (and durability) of a memory foam is primarily determined by its density. In the higher densities … it can be a very long lasting foam.
Latex is a more “instant” foam which doesn’t wrap itself around the body as much. It is also very “point elastic” which means that the load bearing surfaces are very conforming at the load point so the pressure relieving qualities are similar to memory foam. It is easier to move on and has a more open celled matrix than memory foam and so is much more breathable. It comes in two basic varieties which use different methods to manufacture the foam and these are called the Dunlop process and the Talalay process. Latex foam comes in a wide variety of different firmness levels and is the most elastic of all foams which means that in it’s softer versions it is a great comfort layer material and in its firmer versions is also among the best support layer materials. While there is a wide variety inside this category as well, it is a much more lively and responsive material than memory foam and has a feeling of being more “on” the mattress. It is the most durable of all foam categories and will keep the same feel for longer than other types of foam (softens much more gradually).
Finally polyform has a huge variety of different qualities from very low quality to very high quality. Unfortunately … the lower quality polyfoams are easier and cheaper to make in softer versions and are extensively used in even the highest quality mattresses because alll mattresses need some softer foam for pressure relief. The highest qualities of polyfoam (called HR or high resilience foams) approach latex in their properties but are much more expensive than cheaper polyfoams. High quality polyfoam can last for a long time (like high quality memory foam) but lower quality polyfoams … especially when used in the upper layers of a mattress where they are more subject to mechanical stress … can break down very quickly. They are generally the weak link of most mainstream mattresses and as they break down they become softer and less supportive. In the final stages of breakdown they form body impressions (they don’t spring back) even without weight on the mattress but long before this happens they have usually softened so much that they are no longer suitable for sleeping on.
Because of the extensive use of either lower quality polyfoam or memory foam in mainstream mattresses … or even worse … layers of polyfoam or memory foam which are not identified as to their density (meaning the durability is unknown but likely low) … I usually recommend avoiding any mattress which has “unknown” foam or is made by a manufacturer (like the major brands) which do not disclose the quality or density of their foam. The risk of early failure of the mattress is just too high. This is the reason behind some of the guidelines I wrote which you can read here.
Once you have decided on the type of materials you tend to prefer in both the comfort layers and the support layers (more details are in the different pages of the mattresses section of the site) … then it’s time to start testing mattresses. In addition to the more general information in this section … there is also a more detailed article about memory foam here and about latex here.
The first and most important part of this step I would recommend is to find smaller manufacturers that either sell factory direct in your area or who sell wholesale to better sleep shops in the area. You will find more knowledgeable people in these types of outlets that are more concerned with matching you to a mattress that is suitable for your height, weight, shape, and sleeping positions than they are with just making the sale. They are also far more open about what is in their mattresses and actually want you to make meaningful comparisons between different brands and models based on materials rather than doing everything they can to make meaningful comparisons more difficult. They tend to make and/or sell mattresses which have much higher quality at lower prices. In other words they have much better value, knowledge, and service.
Mattresses today are much different from 10 years ago and people who haven’t gone mattress shopping in a decade are usually in for a shock. The brand names really mean nothing (in spite of massive advertising efforts … mostly successful … to persuade people otherwise). The major brands and the chain stores and larger outlets that are effectively in partnership with them in selling mattresses with lower quality materials at higher prices dominate the industry today.
The different sections of the website and many of the forum posts outline some general guidelines which may be suitable for different body types and sleeping positions however the more knowledgeable and service oriented the outlet you are dealing with is … the less you have to know.
So my best suggestion is to gather the level of information you are comfortable with about the different materials used in the comfort and support layers of a mattress (from general to more specific) and then focus on local manufacturing and/or smaller brands which sell factory direct or through better quality sleep shops in the area.
The short answer to this is yes and there are many local manufacturers who will custom design mattresses for each person or side to side for each couple. There are also online manufacturers who specialize in “choose your own layer” mattresses including several members of this site.
Post #7 here (among others if you do a forum search on New Jersey which I’m guessing you already have) will give you some of your better options in New Jersey with a brief description of the types of mattress you will find.
Post #21 here will give you a brief description of the manufacturing members of the site (listed here) which specialize in mattresses which can be shipped across the country. Most of these offer “choose your own layer” or “modular” types of mattresses. As you will see they have different options to customize a mattress in terms of layering and zoning.
Hope this helps … and keep the questions coming if there is anything I have missed or needs further clarification either before or during your mattress testing.
Phoenix