Hi kevine,
Technically the Luxe is softer than the Supreme but the perception of firmness/softness is very subjective and can also vary widely between different people based on body type, sleeping positions, and personal preferences. The firmness or softness of a memory foam mattress can also vary with temperature and humidity or the length of time someone lies on a mattress and it can also vary based on the age of the display model and how much it has been used for testing (whether it is brand new or broken in) but regardless of any of this … if it feels softer for you then that’s what is important regardless of how it may feel for anyone else.
When you have eliminated all the your worse options and are down to finalists that are a comparison between “good and good” and there are no clear “winners” between them then post #2 here may be helpful with how to decide between them. Neither one would be a mistake in terms of quality/value and both have good return policies.
FWIW … making my own “final choice” between my finalists when I purchased my mattress took me longer than all my other research combined (several weeks) as I kept changing my mind back and forth based on small details or differences that in the bigger picture were relatively unimportant but that were the only way I could decide between them … at least without just flipping a coin 
Phoenix