Hi Jean9,
Welcome to the Mattress Forum! 
Latex is certainly a good quality and durable material, and if you have an affinity for the comfort it provides, it can make a very good choice. As you may already be aware, both Foam Sweet Foam and SleepEZ are members of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. They are extremely knowledgeable about latex and different configurations, and if you do decide to purchase from one of these companies remember to ask for your Mattress Underground discount. In this situation, you’re choosing between “good” and “good”, so it comes down to which item might tick off the most boxes on your personal value equation.
I’m sorry to hear about your fibromyalgia. In such a situation, where you are very sensitive to pressure with your fibromyalgia, you may end up preferring slightly softer upper layers on your site of your latex component system, but you’d still desire firmer base layers. Before making a decision, I would recommend a detailed phone conversation with both SleepEZ and Foam Sweet Foam and let them know of your specific concerns for both your husband and yourself. These companies will be able to provide the best advice on how their componentry will work with your particular needs and based upon the information you provide will recommend configurations that they honestly think will suit your situation. As Foam Sweet Foam says on their site, “If you are in doubt, give us a call. We have been doing this a long time and are usually right. Our goal is to get the purchase correct the first time.” The same would go for SleepEZ. I know that you desire to “get it right” the first time, but at least with both of systems you’re considering you do have the ability to fine tune and customize in the future.
As for fibromyalgia, a forum search on fibromyalgia (you can just click the link) will bring up more comments and feedback from others that are in similar circumstances that may be helpful (but it will also confirm that there isn’t a single “best” combination of materials that will work for everyone with fibromyalgia).
Overall, the two basic functions of a mattress are to support and to provide comfort (you can read more about that here if you like), with alignment being the first priority and then comfort coming second. Usually when people are referencing a “firm” mattress to you, they’re referring to the level of deep support, which you certainly would want to be robust. But that doesn’t necessarily mean that the mattress you choose would have to feel like a board on top. The uppermost layers within a mattress will be the ones that have the greatest impact upon your comfort, and the deeper layers will primarily be concerned with alignment and support.
Having a higher BMI generally requires firmer materials (in the support layers especially), and latex would be a quality material that could provide this level of support. The same overall guidelines apply with higher weights through PPP (Posture and alignment, Pressure relief, and Personal preferences), along with the use of high quality durable materials (like latex) that will maintain their feel and performance for longer periods of time, are the way to make the best choices. Heavier people in general will need firmer and thicker comfort layers and firmer support layers than those who are lighter, and the four layer system you’re considering would generally a good choice. Post #3 here has more information and suggestions about heavier weights that is worth reading.
I hope that helps, and I’ll look forward to learning about any decisions you make.
Phoenix