Hi Harpergigs.
Welcome to our Mattress Forum! 
Hopefully, ArionIne is still subscribed to this topic and will see your question and give you a four years account of her experience with the mattress.
Having comfort layers on both sides of a mattress can certainly extend the useful life of a mattress (assuming you rotate and flip it on a regular basis) compared to the same mattress with the same comfort layers on only one side of the mattress. Having said that … if you are comparing two different mattresses that have different designs and use different materials and components in the support and comfort layers then someone sided mattresses may also last longer than some two-sided mattresses depending on the quality and durability of the materials and components in each of the two mattresses you are comparing so this would only apply to two mattresses where the only difference between them was that one of them had an additional comfort layer on the bottom of the mattress. There is more about the pros and cons of a two-sided mattress in post #3 here.
I am not very sure what you’ve read since finding us, but just in case you haven’t read it yet the first place to start your research is the mattress shopping tutorial here(there is a condensed version of it at the end of that post) which includes all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly know how and why to avoid the worst ones. Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort”, firmness, and PPP that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well he will sleep), durability (how long he will sleep well), and the overall value of a mattress compared to your other finalists based on all the parts of your personal value equation that are most important to you (including the price of course and the options you have available after a purchase if your choice doesn’t turn out as well as you hoped for).
When assessing any product, also be sure that you find out information listed here so you can compare the quality of the materials and components to the durability guidelines here to make sure there are no lower quality materials or weak links in a mattress that would be a cause for concern relative to the durability and useful life of a mattress before making any purchase.
I look forward to hearing about your decision or about any of the insights that your testing experience with Best Mattress products might bring you.
Phoenix