The lighter wire and fewer turns are compensated by using a higher coil count even though there may be less steel used than in bonnell types. This Marshall type coil is usually not tempered and of a very thin gage wire. Coil counts are very high due to the relatively weak spring structure. The availability of so many different spring units makes your choice more difficult. Very high coil counts, some over 800 coils in a full size mattress, sound great, but may offer less support than a lower coil count in another coil type.
Buyers have more to consider than just the coil count.
You can't see the coils inside the mattress and probably couldn't tell why one is better than the other even if you could. Our advice is to let your back and your dealer help. Lie down on the mattress and consider how it feels. Does it support the whole length of your body or sag under the heavier areas. Lie there for a while, your body will let you know if the sleep set is right for your needs.