Hi danliang137,
I’m happy to speak to the quality and value of a mattress but as you can see in mattress firmness/comfort levels in post #2 here … comfort choices should always be based on either your own personal testing and experience or more detailed conversations with a manufacturer or retailer who knows their mattresses better than anyone else.
The most important part of a mattress in terms of quality and durability are the comfort layers and the foams used in the Cool Comfort are a little thicker and a little higher quality (4" of a combination of 4 and 5 lb memory foam vs 3.3" of only 4 lb memory foam). It also has options to rearrange or even exchange layers which means it has more flexibility of design and gives you a chance to do some fine tuning after you have slept on the mattress.
I think you may have misunderstood my comment (or I didn’t express it well). I was suggesting that with your weight you may be better off with higher density memory foam (5 lbs or higher) or at least thinner layers of lower density 4 lb memory foam in combination with higher density memory foam because of the higher risk of foam softening having a bigger effect with thicker layers of 4 lb memory foam. The comment wasn’t about the total thickness of the layers, just about the amount of 4 lb memory foam in the mix.
While only your own experience can know for certain … a combination of 4 and 5 lb memory foam may give your girlfriend a couple of inches of softer 4 lb memory foam to sink into for her for her lighter body type before she starts reaching firmer layers and with your higher weight you would sink more deeply into the firmer and higher density / durability 5 lb memory foam which would act like more of a comfort layer for you and would be more “supportive” for her. You would also have the option of putting the 5 lb memory foam over the 4 lb memory foam if you both preferred the feel of that combination and you also would have the choice of re-arranging the support layers to fine tune the support as well. While it’s a little more costly … it would provide you with more options for customizing the mattress after you receive it.
Of course the most important part of making a choice between them would be a more detailed conversation with Rocky Mountain Mattress.
Phoenix