Hi Colleen,
Welcome to the Mattress Forum! 
While I can certainly help with “how” to choose … It’s not possible to make specific suggestions or recommendations for either a mattress or combinations of materials or components or a specific comfort level, because the first “rule” of mattress shopping is to always remember that you are the only one that can feel what you feel on a mattress and there are too many unknowns, variables, and personal preferences involved that are unique to each person to use a formula or for anyone to be able to predict or make a specific suggestion or recommendation about which mattress or combination of materials and components or which type of mattress would be the best “match” for you in terms of “comfort”, firmness, or PPP or how a mattress will “feel” to you or compare to another mattress based on specs (either yours or a mattress), sleeping positions, health conditions, or “theory at a distance” that can possibly be more reliable than your own careful testing (hopefully using the testing guidelines in step 4 of the tutorial) or your own personal sleeping experience (see mattress firmness/comfort levels in post #2 here).
When shopping online and you can’t test a mattress in person, then the most reliable source of guidance is always a more detailed phone conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and who can help “talk you through” the specifics of their mattresses and the properties and “feel” of the materials they are using (fast or slow response, resilience, firmness etc.) and the options they have available that may be the best “match” for you based on the information you provide them, any local testing you have done (like the Tempurpedic models you tried) or mattresses you have slept on and liked or other mattresses you are considering that they are familiar with, and the “averages” of other customers that are similar to you. You can also learn about any exchange, return, or any options they have available to customize a mattress after a purchase can help lower the risk of an online purchase. A good online retailer or manufacturer will generally make suggestions that they honestly believe have the best chance of success based on the information you provide them when you talk to them.
I would imagine that Nest would recommend either their Soft or their Medium in the Alexander Signature based upon the brief information you presented here, but you’d definitely want to have a more detailed phone conversation with them. They also offer a 100 night trial should things unfortunately not turn out as well as you had hoped for. Nest Bedding is a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. I certainly wouldn’t hesitate to purchase a mattress from them.
While other people’s comments about the knowledge and service of a particular business can certainly be very helpful … I would always keep in mind that you are the only one that can feel what you feel on a mattress and I would be cautious about using anyone else’s suggestions, experiences or reviews on a specific 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. In many if not most cases they can be more misleading than helpful because a mattress that would be a perfect choice for one person or even a larger group of people in terms of comfort, firmness, and PPP may be completely unsuitable for someone else to sleep on (even if they are in a similar weight range). In other words … reviews or other people’s experiences 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).
I’ll look forward to your comments and any decisions you might make moving forward.
Phoenix