Hi MystrE, and welcome to the Mattress Underground 
As you have found,shopping for a new mattress can be very frustrating. As any mattress you and your wife finally decide on will be based on your Stats (height, BMI, sleeping position(s) and any underlying health concerns) - thanks for providing some of those - and your PPP (posture & alignment, Pressure relief, and Personal preferences).
If you have not already, you should take a look at the Mattress Shopping Tutorial, to get an idea of what you are looking for in a mattress retailer, as well as the Mattress Specifications You Need To Know and compare these to the Mattress Durability Guidelines to see how certain materials will last over time. in general, memory foam, of insufficient density can prematurely ‘wear out’, causing worn areas as you’ve seen, and even ridges and divots which can cause muscle aches and pains, and even alignment issues.
My wife and I went to a local Mattress Firm and tried the Tempurpedic Adapt Medium and Pro Adapt Medium Hybrid along with a Sealy Posturpedic Plus Albany medium hybrid and Serta I series medium hybrid. I liked the Pro Adapt Medium Hybrid but the price is out of our budget plus I think I would have to get a Tempurpedic adjustabtle base which really puts that out of our budget. I preferred the feel of the Sealy Posturpedic Plus over the Serta but neither mattress gave me any confidence concerning their durability.
Your old Serta iComfort is a memory foam & gel memory foam mattress. Like most of the ‘big brands’ their lack of details on the components construction and even layer thickness makes it near impossible to assess for any sleeper except in the most general terms. Many times, a lack of transparency in components can hide an excess amount of inexpensive foam ‘filler’…As you both have higher BMIs, any memory foam you choose should have a higher density to maintain support, especially with your neck fusion. We suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
How different is sleeping on latex vs. memory foam/memory foam hybrid mattress? The safe route comfort wise is to go with another memory foam or memory foam hybrid from the many different mattress manufacturers available online.
Latex has a very specific feel that is unlike memory foam. Talalay latex is considered ‘bouncier’ whereas Dunlop is more ‘settled’ (think of the difference between angel food cake and pound cake). If possible you might want to try latex or latex hybrid mattresses in a local showroom, just to see if you like the feel. Latex is one of the most durable mattress materials you can have and if you and your wife like the feel and can find a comparable comfort level, you will have a mattress that will last many years.
we have always purchased mattresses locally and ordering online is somewhat a leap of faith.
How are trusted members vetted for this site? If I do order a mattress online, I want to do so from a company that stands behind their products and have great customer service should an issue arise.
The Trusted Members of the site have all gone through a rigorous vetting process; these are all smaller manufacturers and retailers who take great pride in the products they carry - their reputation rides on every sale. They all offer quality sleep solutions and have great experience helping consumers find their ideal mattress, with excellent customer support and generous exchange and return policies. It can be somewhat disconcerting to make such an important purchase online, but our Trusted Members carry a variety of different sleep systems at various price points, and they are all very transparent as to what goes into their products. , and have years of experience in helping consumers find their ideal mattress. I think you will be pleasantly surprised with the process if you choose to go that way.
~ Basilio