Hi nc11383,
I would certainly avoid 3 lb memory foam when you are in the 200 lb range. You can see my thoughts about Bedinabox in post #2 here.
IFD is not a particularly meaningful specification with memory foam because the IFD changes with heat, humidity, and the length of time the foam is compressed. Specifications such as temperature response, density, recovery time, and layer thickness along with the layers above and below the memory foam layers would have much more to do with the feel and performance of a specific tye of memory foam than IFD. Higher density memory foam tends to be more “supportive” than lower density memory foam because it has a higher compression modulus (how quickly it gets firmer with compression) but this will also vary between different types of memory foam.
I would avoid choosing a mattress based on IFD specs which are only one part of a mattress design that provides good alignment for different body types and sleeping positions. It would be somewhat like choosing which foods to eat based on vitamin D content alone.
One of the two main functions of a mattress is good alignment (the other is pressure relief) so that your spine and joints are all in the middle of their neutral range of motion. The best way to visualize this would be to imagine yourself floating in the air with “good posture and alignment” in the position (or positions) you normally sleep in and then slowly being lowered onto the mattress. Some parts of your body would need to sink in more and others less so that they all “stop” sinking in at the same time. Some parts of your body are heavier and need to be “stopped” with firmer support in deeper layers so they don’t sink in too far while other parts of the body that are lighter and/or wider need softer or thicker comfort layers so that they are “allowed” to sink in more. You also need to sink into a mattress enough so that the recessed parts of your body (such as the waist or small of the back) can make firm enough contact with the mattress to prevent any sagging underneath them.
A mattress that is too firm or has too little softness or thickness in the comfort layers can result in poor alignment (because the support isn’t even along the entire length of the body and some areas of the body may be completely unsupported). A mattress that is too soft or where the comfort layers are too thick can result in poor alignment because the heavier parts of the body (the pelvis in particular) can sink down too far before they are “stopped” by the firmer support layers.
Each manufacturer will have more knowledge about their own mattresses and how well they “match” different body types, sleeping positions, and personal preferences “on average”. When you are considering an online purchase … a more detailed conversation with them that includes more specific information about your own body type, sleeping style, preferences, and your personal experiences in testing mattresses will be the best source of guidance about which of their mattresses may be the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences).
The memory foam they use in their mattresses are higher density than the BIAB mattresses so they will tend to be more durable and probably more suitable for higher weights as well (see the memory foam guidelines in post #10 here).
Again though … they know more about their mattresses than anyone and also have the most interest in helping you make the most suitable choice (to avoid a return) so I would tend to follow the guidance they provide after a more detailed conversation with them.
Phoenix