Hi jodebg,
I would tend to avoid trying to define “best” in terms of a single element of the construction of a mattress because it will lead you into the type of overanalysis that is a never ending rabbit hole filled with too many different opinions and variables in the answers and you will be faced the need to learn the knowledge it would take to become a mattress manufacturer to answer all the “it depends” that are part of any question like this. “Best” also depends on the specifics of the application and on the overall details of the construction of the mattress and on the personal and subjective factors that are involved in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). It would be like trying to define the “best” article of clothing by defining which type of seam stitching is best. A more effective approach is to identify any potential weak links in a specific mattress.
Having said that … in general steel edge support constructions are stronger and more durable than foam but if a foam surround is high quality (I would suggest 1.8 lbs and higher) and is well balanced with the rest of the mattress design in terms of firmness and well glued and attached to the spring unit so it doesn’t delaminate or splay out it can also be a suitable and durable choice that can last for the life of the mattress.
Yes … a two sided mattress will last significantly longer than a similar one sided mattress in an apples to apples construction (same build and design except for the extra padding on the other side) as long as you flip it regularly but it also has some limitations in terms of design. There is more about the pros and cons of each in post #3 here.
Phoenix