Hi lovingthebay,
The guidelines I use for a memory foam mattress are in post #10 here. Of course these are just for quality/value comparisons and have nothing to do with which mattress may be suitable for you so I would also make sure you are dealing with a merchant who has the experience and knowledge to help you make the most suitable choice … especially when you are dealing online where your recourse if you make a bad choice may not be good (in which case whatever you spend will have been wasted).
None of the LinenSpa meet the guidelines I use so I would exclude them.
The Sleep Innovations would depend on knowing the density of all the foam they use because some of these use memory foam that is lower quality than I would consider.
I also wouldn’t consider the Dynasty because they also don’t meet the guidelines and they have also listed the density of their foams incorrectly (see post #5 here) so there is no way to know for sure what they really contain.
Some of the The better sources for online memory foam mattresses I’m aware of are listed in post #12 here.
Phoenix