Hi TomD,
The “best” and “least risky” foundation would have no flex at all and would be similar to putting your mattress on the floor (outside of the fact that a floor or a solid surface foundation would restrict the airflow under the mattress which would have its own risks … see post #10 here). This would allow the materials in the mattress itself to do all the compressing the way it’s designed to do. If a foundation has more flex in it or if it uses weaker or more flexible wood then it can allow the heavier parts of your body to sink down more deeply which can effect your alignment. It could also develop a sag in the foundation itself if it is being compressed too much on a regular basis. While this isn’t likely to happen to a degree that most people would notice on any of the foundations you are considering … the risks would be higher with higher weight ranges or with people that are more sensitive to smaller differences between two sleeping systems. There is no way to quantify how much of a difference it would make for any specific person over the longer term so it would be a choice that would be based on your “risk tolerance” and “best judgement”. For some people that were more comfortable with lower levels of risk or that were less cost sensitive it would be worth it … and for others it may not be.
Phoenix