Hi Halifax,
While 40 ILD is certainly firm … the firmness rating is taken at 25% compression and if the layer is compressed less than this is would be softer so it would really depend on how your body type interacts with the mattress as a whole. With the “bouncing” test and their ability to make adjustments to the mattress its unlikely that it will come back to “haunt” you if you test it carefully and with a mattress that is a little bit too firm you can always add a topper to add some extra pressure relief so with Fox’s ability to make adjustments to the mattress and/or add a topper you still would have good options but there is little you can do to “fix” a mattress that is too soft without removing and replacing layers instead of adding to them.
I don’t have any personal experience with it (OMF doesn’t have any stores near me) but again … only personal testing and experience can really know if a mattress is suitable and there is no “formula” that can know this based on specs. The firmness of polyfoam is also measured differently than latex so 30 ILD would be firmer than the same ILD with latex (see post #6 here). You can see my thoughts about the new Serenity in post #3 here. You can also see my thoughts about a latex/polyfoam hybrid vs an all latex mattress in post #2 here.
ILD is not a “quality” spec though so when you are testing a mattress locally it’s not an important factor to take into account because your body will tell you more about whether a mattress is a suitable “match” for you than any “comfort specs” such as ILD. Neither of the mattresses you are considering have any obvious weak links in their construction so any final choice between them would be a choice between “good and good”.
Phoenix