Hi Snuggles2013,
Just in case you haven’t read it yet … the first place I would start your research is the tutorial post here which has all the basic information, steps, and guidelines that can help you make the best choices.
The King Koil (Comfort Solutions) XL line generally uses higher quality and more durable materials than their other lines (you can see an example of one of their XL mattresses here with my comments) but a mattress is only as good as its construction and the quality of the materials inside it so without knowing the specifics of each layer there is no way to make meaningful assessments of a mattress or make reasonable comparisons with other mattresses.
Higher weights will soften and breakdown any material or component faster than lower weights so the durability of the materials and components becomes especially important … especially in the upper layers of a mattress which are generally the weak link of a mattress. There is also more information in the first part of post #2 here about ways that can help accommodate a couple that has very different body types or specific needs and preferences.
Bay Bed and Mattress is a member here that specializes in customized pocket coil / latex mattresses that have individual components, can be shipped across the country, and can include a side to side split construction.
I don’t keep a record of individual mattresses that may be available in each area but if you let me know your zip code or city I’d be happy to let you know of any possibilities I’m aware of in your area.
Phoenix