Hi dottie,
You are fortunate to be in Phoenix as there are several very high quality and knowledgeable factory direct manufacturers nearby including two of the members of this site (meaning I consider them to be among the best of the independent manufacturers in the country) which offer great quality and value and perhaps more importantly have a lot of knowledge in fitting a mattress to an individual’s needs and preferences. This thread and post #4 here should be very helpful.
I would not order online when there are such great options available locally that have better quality and value and that you can personally test. If you want to keep your choices down for the sake of time … I would focus my efforts on the two manufacturers that are members here.
In cases like yours … memory foam can be very tricky because not only is pressure relief very important (which memory foam like latex does very well), spinal alignment can be even more important and the many different types of memory foam layering options can mean the difference between a mattress that can work well for you and a mattress that doesn’t work well at all (usually too thick in the comfort layers or support layers that are not the best for you). Memory foam is only a pressure relieving material and if the layers are too thick or otherwise unsuitable for your particular needs or the support system under the memory foam isn’t what you need … it can cause more harm than good.
The Gel memory foams are also an emerging category and there are several different types, some of which are better than others. In general … I would tend to avoid the types of gel memory foams that use gel particles mixed in the memory foam (like the icomfort) as particles added to memory foam can weaken the memory foam. There is an analysis and review of the iComfort lineup in post #11 here (none of which have good value) and a brief overview of the different types of gel memory foam options in post #26 here.
Hope this helps … and I only wish that the rest of the country had local choices that were as good as yours.
Phoenix