Hi sidesleeper020,
With a $400 credit I think that even “throwing it away” you would end up with better quality/value with either of the choices you are making.
When you are down to making final choices between “good and good” (see post #2 here) then it’s really up to your own personal criteria and all the objective, subjective, and intangible factors and tradeoffs that are part of a mattress purchase as a whole that are most important to you. In other words it’s not so much a choice between “better and worse” and more a matter of which is best for “you” based on your own “personal value equation”.
Either way, given the amount of credit available to you … you would be making a better choice than the alternative you were considering IMO 
Phoenix