Send an email to Costco customer service with your questions. They send it to the supplier and get back to you with an answer.
My impression is that the Sleep Science is described as firm and Spring Air is their version of a softer feel or more plush feel mattress with Latex. Maybe someone here on the forum can comment if that is not correct.
We have last year's version of the Costco Spring Air latex mattress with the soy-based layer. I've been very happy with this bed. It is fine as is, but I already had a convoluted latex topper (enclosed in a wool and cotton cover) so we have that on top of the mattress for a plusher feel.
YMMV.