Hi AdamRB,
There is more about what I call “simplified choice” mattresses in general in post #1 here and there is more specific information about the BestMattressEver and Leesa in post #2 here in the same topic.
As you can see the BME uses higher quality and more durable materials and the 2" of 3 lb memory foam in the Leesa is lower quality/density than the guidelines I would normally suggest and could be a weak link in the mattress.
The medium would cover the largest part of the bell curve that would be suitable for the largest percentage of people and would be the most common choice but when you can’t test a mattress in person then the most reliable source of guidance is always a more detailed 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 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. They will know more about “matching” their specific mattress designs and firmness levels to different body types, sleeping positions, and preferences (or to other mattresses that they are familiar with) than anyone else.
Phoenix