Hi jkozlow3,
I don’t think it would be possible to answer your question specifically without more sophisticated testing equipment in a lab that was able to measure how you sleep on different types of mattresses but I think in very general terms you probably answered your own question.
Coils that are connected together with helical wires (Bonnell, Offset, and Continuous coils) share the load between the coils more so they offer more resistance to pressure than more contouring types of support systems (pocket coils and many foam support systems) so they will tend to “hold up” the heavier parts of the body very effectively. There are many people that do very well on these types of “connected” coils because their particular weight distribution (relative weight distribution can be more important than just weight) and body shape benefits from this type of coil system to help them keep their spine and joints in good alignment without part of their body sinking down too far. Of course the thickness and firmness of the comfort layers above the coils and the specifics of the coil itself will also make a difference (there is a wide variety of each type of coil) and comfort layers that are too soft or too thick can also cause alignment issues from the heavier parts of the body sinking down too far before being “stopped” by the firmer coils but in very general terms I think that the reason “why” is because of the type of deeper support that these types of coils offer and their ability to “stop” the heavier parts of your body from sinking down too far. There is more about the different types of innersprings in this article and in post #10 here.
I’m assuming that you are using Sealy and Serta as examples of each type of spring because as you probably know I wouldn’t suggest buying either a Sealy or a Serta because regardless of the types of coils they use they both tend to use lower quality materials in the layers above the coils which would be a weak link in a mattress. There are many other smaller manufacturers that use both types of coils (although the offset would be more common) that use higher quality materials above the coils so you would have the best of both worlds (the more supportive coils you do better with and more durable comfort materials). Don’t forget that the weakest link of a mattress is generally in the comfort layers and not the support system.
In terms of how they compare … I would tend to choose an offset coil (knotted would be stronger and more “supportive” than an open end offset such as an LFK unit) even though they will tend to be more costly. In very general terms again they will both have have similar support but offset coils tend to use more steel (more turns in the coil and lower gauge wire) and can generally contour a little better and are a stronger unit overall and will possibly be less noisy as well. The only reason I would tend to choose a continuous coil would be for budget reasons.
There are some very high quality offset coil mattresses and you can also find them in two sided versions which will be more durable yet. The biggest compromise with connected coils are that they tend to transfer more motion than other types of support systems.
Phoenix