Hi rb,
There are too many variables and unknowns to use “theory at a distance” to predict which mattress you will do best with but mattress firmness/comfort levels in post #2 here has more about how to make the best comfort choices. If you are following the steps listed in post #1 here one by one then IMO you are on the right track 
The Regalis is very similar to the Tempurpedic Rhapsody and if you talk with Select Foam they would suggest that you use the Rhapsody as a guideline. I would make sure that you use the testing guidelines in the “read first” post to test carefully and objectively for PPP (Posture and alignment, Pressure relief, and Personal preferences) on the Rhapsody and if it works well for you then the Regalis would likely make a good choice as well and is much better quality/value.
Phoenix