Hi handsoap,
The density of the base layer in the Select Foam mattresses are probably higher (Sleep Science doesn’t list the density of their base layer) but the base layers aren’t usually the weakest link of a mattress so outside of any uncertainty that is due to the compressed shipping they use similar quality/density memory foams so “in theory” they would be closely comparable in terms of durability with a possible slight advantage to Select Foam.
The comments you are probably referring to about Select Foam are probably the link to a caution about them that is included in the comments in the online memory foam list here which talks about some of the customer service issues that some of the members here have experienced with them. NOTE ADDED: Because of ongoing customer service issues and delays Select Foam’s membership has been terminated and I would read the warning here before considering them.
Phoenix