Hi eevee188,
I think dn covered most of the basic tradeoffs between one and two sided designs in the previous post but you can read more in some of the posts he mentioned including post #2 here and post #2 here and post #2 here and some of the history behind one sided mattresses in post #3 here.
There is more about some of the other factors that can contribute to durability and the useful life of a mattress relative to different people in the durability guidelines here as well.
There are also other ways besides a two sided design that can decrease the long term cost of owning a mattress (price vs useable life) outside of using higher quality and more durable materials.
One of these includes a mattress that has individual components or layers with a zip cover that can be replaced individually instead of having to replace the entire mattress. This would normally involve the top layer because a mattress will tend to soften and break down from the top down. Individual layers can also be flipped individually. This also has the advantage of being able to make changes to your mattress either to fine tune the feel and performance after a purchase of to make changes down the road if your needs and preferences change.
There are also many smaller local manufacturers that will open up a “finished” mattress and replace one of more of the layers either after a purchase if it needs to be fine tuned or to rebuild it down the road if one of the layers needs to be replaced.
Phoenix