Hi beth819,
The NXG 600 is a similar idea to the 575 except the combinations on top are a little different (latex in between memory foam instead of memory foam in between latex). I agree with you though that I would never consider purchasing of any of these because the value is poor compared to many other better options.
Post #11 here includes my thoughts on all of the iComfort lineup. In essence … considering the materials and the price … they (like almost all major brand mattresses) are also poor value IMO.
Phoenix