"What is the major difference between
the 7 and 9 ply sets?"
The number of layers or plies should be a matter of preference -- and not quality.
Our 7-ply (KT17ULTRA) set was originally manufactured for use on induction-top stoves. This set has a layer of carbon steel specifically for use on the induction-tops (cookware that is used on induction-tops should have a strong ability to attract a magnet). However, the set may be used on any stove-top, including gas.
Do you need a layer of carbon steel in the construction of your waterless cookware set?
No, you do not, that is unless you own an induction-top stove, then the KT17ULTRA set is the set you will want to purchase.
The extra layers in the 9-ply sets (KT915 and KT17) consist of extra layers of bonding agents. The difference between the weights per pot in all three of our sets is so slight that, if you were blindfolded and held a pot from the KT915 and the KT17ULTRA sets in each hand, you would not be able to tell the difference.
Some companies advertise their cookware as being "multi-plied" (the plies seem to be growing each day). All that means is that they've probably added more layering than what is normal, or necessary. All of this is more about marketing than it is cooking.
There are four factors that must be present in a set of cookware in order for it to be called "waterless."
If all four factors are present, then you have a set of waterless cookware. At least 5-plies of construction is considered superior to less.
Surgical stainless steel is the perfect material for waterless cooking, and a layer of material placed in the middle of the bottom of the pot which causes fast and even heating possible is all that is needed to cook using the waterless method.
Neither a higher price - nor adding more layers than necessary - can make waterless cooking "more possible."