Hi zfeldman,
ILD by itself won’t tell you how soft or firm a foam will feel to you or how likely you are to bottom out on it. The surface softness of the foam will be more connected to the ILD/IFD (it only refers to the relative firmness when a 4" square of foam is compressed by 25% of its thickness and not how soft or firm it will be if you compress it more or less than that) and how likely you are to bottom out on a foam will be more directly connected to the compression modulus/support factor of the foam (which is the rate that a foam becomes firmer as you compress it more deeply) and both of these together (along with thickness and the point elasticity of the foam) are connected to how soft and/or supportive the foam will feel overall (see post #4 here).
I don’t know your body type or the specs of the specific Quaalux foam you are considering because Quaalux is a “family” of foams (although it’s a good quality and durable material) and I have little experience with camping pads (outside of the Therm-a-Rest I used to use when I was doing long distance bicycle touring many years ago) so your best source of guidance would probably be a knowledgeable supplier that will know more about the specific properties and specs of the foams they sell and how they combine together in different applications than I would.
Depending on your body type … I would tend to use a little lower ILD for “comfort” but choose a foam that has a higher compression modulus/support factor (to prevent it from bottoming out) for “support”. Of course higher quality foam with better “specs” are also more costly.
The difference between 35 and 41 ILD would depend on the other specs that can affect how soft a foam “feels” when you sleep on it but if all the other specs were the same then the difference would be relatively small.
The component list here includes some of the better online sources for polyfoam that I’m aware of. There are also some comments about their relative prices in post #4 here and post #6 here.
You’ve probably read these but just in case you haven’t I would read this post and this post and this topic (about their polyfoam and sources) and this post (presumably from a past employee) before buying anything from Foam by Mail (AKA FBM and Foam Factory) or considering them as a reliable supplier … especially if the specific properties of the foam you are buying are “application critical” and would make a significant difference to you.
Phoenix