Hi Lisaj950,
There is more about the new “breed” of mattresses that only offer a single firmness choice in post #3 here but I would be cautious with the Leesa because their middle layer is 2" of 3 lb memory foam which is a lower density material than I would be comfortable with in terms of durability.
There is no “standard” definition or consensus of opinions for firmness ratings and different manufacturers can rate their mattresses very differently than others. Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that feels firm for one person may feel like “medium” for someone else or even “soft” for someone else depending on their body type, sleeping style, physiology, what they are used to, and their individual sensitivity and perceptions. Different people can also have very different opinions on how two mattresses compare in terms of firmness as well. This is all relative to each person’s perceptions and circumstances and the only reliable way to know how firm or soft a mattress feels to you or how it compares to another mattress (regardless of how firm or soft it may feel for someone else) will be based on your own personal testing or sleeping experience.
Having said that there may be some members here that have tried both of them that see your post and can share their opinions as well although it’s not likely that there are many people that have tried both of them.
Phoenix