Hi bean_counter,
Latex is the most durable of all the foam materials (polyfoam, memory foam, latex foam) regardless of which manufacturer uses it in their mattress. You can also read more about some of the other factors that have an effect on the durability of a mattress in post #4 here.
A two sided mattress will be more durable than a one sided mattress regardless of the type of material but the tradeoff is that there is also less design flexibility because the firmer support layer is in the middle of the mattress and there is less “room” to add softer materials on both sides because if there is too much soft foam on the bottom of a mattress it can affect alignment/support.
Phoenix