When buying a futon mattress, especially ones made of memory foam, it is important to keep a few factors in mind to make sure you get one that meets your need.
Price: In general the more you spend the better the comfort and support will be. Memory foam may all look the same but there are varying qualities. Make sure you understand that a cheaper memory foam futon may not give the support and comfort to someone who plans to sleep on it every night.
Use Case: How do you plan to use this futon? If you will be sleeping on it every night we highly suggest buying a futon frame and a separate high quality futon mattress. If you will mostly be using it as a couch that will sleep occasional guests, you could get away with purchasing a cheaper all-in-one futon from Amazon.