Hi scoaste,
This would be a completely different category of mattress than the other three mattresses you are considering (see this article). It uses a Bonnell innerspring for a support core and includes a microcoil, polyfoam, memory foam, and synthetic polyester fibers in the comfort layers.
There is more information about Saatva mattresses in post #2 here and in post #1 here and the rest of the topic and a forum search on Saatva (you can just click the link) will bring up more comments and feedback about them as well.
While Saatva would be a “better than average” quality/value choice compared to many of the mainstream mattresses made by the major manufacturers (which I would avoid completely) … some of the materials they are using are less durable than I would be comfortable with and for most people they wouldn’t be in the same “quality/value” range as many of the other options that use higher quality and more durable materials that most of the members here would know about as a result of the information on this site. I would also be cautious about making a purchase based on some of “marketing exaggerations” on their site.
That would depend entirely on how you define “twice as good” if you are referring to comfort and PPP because this would be purely subjective. It would be like saying this meal tastes “twice as good” as that meal. It certainly wouldn’t be twice as durable.
You can read more about edge support with foam mattresses in post #5 here and the posts it links to.
The only way to know whether the edge support for a mattress would be “good enough” for you will be based on your own personal experience. It would be fine for “most” people but there may also be some people that would prefer firmer edges.
While other people’s comments about the knowledge and service of a particular business can certainly be very helpful … I would also keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about about using other people’s experiences or reviews on a mattress (either positive or negative) or review sites in general as a reliable source of information or guidance about how you will feel on the same mattress or how suitable or how durable a mattress may be for you and in many if not most cases they can be more misleading than helpful. A mattress that would be a perfect choice for one person or even a larger group of people may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews in general won’t tell you much if anything about the suitability, quality, durability, or “value” of a mattress for any particular person (see post #13 here).
Having said that … hopefully some of the members here that own any of the mattresses you are considering will see your post and share their comments.
Phoenix