I am a senior and a side sleeper and I am considering a sterns and foster lux estate Cassatt luxury plush mattress and I am also looking at a Restonic comfort care plush hybrid and wondered how durable these mattresses are
Hi forbush.
Welcome to our Mattress Forum. 
Without your sleep stats (height, weight, sleeping position) and the specifications of the mattresses in question (including foam densities) it is hard to make an evaluation in terms of durability.
We do have these Durability Guidelines to help any consumer decide whether or not a mattress is durable enough for them.
What is it that concerns you (or interests you) most about these two mattresses?
NikkiTMU