Hi geobix,
First of all … thanks for the feedback on Country bedding :). I’ve linked this thread to a couple of the other posts that mention them.
Your questions are a little more complex than they may seem at first.
First of all regarding foams that have added gel. Gel is a heavier material than either polyfoam or memory foam so when it is added to a foam, it makes the resulting hybrd foam denser. A 4 lb gel foam in other words means that the base memory foam that was used to add the gel was less than 4 lbs (called the unfilled density or polymer density of a foam) and how much lower it was would depend on the percentage of gel that was added.
In addition to this … there are various ways that gel can be added to a foam which you can see in post #26 here.
Gel is a very durable material but as you can see there are various ways to incorporate it into other foams so the effect on the foam durability would depend on how the gel was added and on the polymer density of the original memory foam. Because there are so many variables and because gel foams are an emerging category … the most accurate way to know the relative durability of a gel foam would be the durability testing that was provided to the mattress manufacturer (if they specified that they wanted it). In general though … if it is not a “particulate” type of gel foam … then the odds are fairly good that the gel foam would be more durable than the same type and density of memory foam without the gel added to it. If the gel was added as particles … then the odds are that it could make the memory foam less durable.
Other than this though …the thickness of a material is a big part of how much any foam softening will affect the overall feel and performance of the mattress. If the layers are thinner (as in your case where the gel foam is only in a 2" layer) … then foam softening will have less of a negative effect than if the layers were thicker (like 3" or 4"). Heavier weights will also lead to more rapid foam softening than lighter weights and this will also be offset by the fact that your mattress can be flipped which will add substantially to the durability of the gel foam. As you mentioned … thinner comfort layers also tend to lead to better support because you are closer to the support layers of the mattress.
Because there are so many variables involved … I would rely on the experience and any specific information the mattress manufacturer may have to make a comparison with other memory foams. If the gel memory foam is not a particulate, because the layers are thinner, and because the mattress is two sided … this mattress would be considerably less risky than it would be with particulate gel or with thicker one sided layers using the same materials. While you have good reason to be cautious with your weight … you also have good reason to be optimistic.
Next is the issue of temperature regulation because the temperature of a material when you put your hand on it (thermal conductivity) is only one of several factors involved in temperature regulation while you are sleeping. There is more about this at the end of post #4 here. In addition to this … there are many different variables and technologies involved in making different memory foams sleep cooler and other factors and layers of your sleeping system (ticking/quilting, mattress protector, sheets and bedding) and external conditions will also play a significant role in how cool you sleep.
All of this is to say that the “non gel” 5 lb memory foam may well be more durable and could also cool well enough for someone depending on the other cooling technologies, materials, and fabrics in the mattress and on it’s construction (again thinner comfort layers will tend to be cooler than thicker layers).
So your question is more complex than you may realize and the truth is that the manufacturer themselves may be in the best position to answer it based on the experience of their customers that have tried both and on the information they have about the gel foam and the rest of the components.
If I was in your shoes and the 2/6/2 two sided construction construction worked for you with both types of memory foam … then I would tend towards the higher density memory foam as long as it had a similar feel and I was satisfied that it was either one of the more breathable types of “non gel” memory foam or that there were other cooling components in the mattress that sleeping temperature wouldn’t be an issue. If I tended to sleep hot and this was a bigger concern than durability or the “uncertanties” involved in the gel foams … then I would tend towards the gel foam because of the other factors that make it less risky in this case (assuming it is not a particulate).
I know that this was probably more detailed than you wanted to hear but your “every night” and long term experience is important and with a little more knowledge about the many variables that are part of your durability and temperature questions … you will be in a better position to make a choice based on “educated intuition” with the help of any information the manufacturer can supply.
Thanks again for the additional information about Country Bedding. Regardless of your choice … it certainly seems that they are producing some good quality and value mattresses 
Phoenix