Hi ramseyarthur,
Mattress warranties only cover defects in the materials and they don’t cover the gradual (or more rapid in the case of lower quality comfort layers) loss of comfort and support that comes from foam softening that is the main reason that most people will need to replace their mattress. In other words warranties have little to do with the durability or useful life of a mattress or when you may need to replace it and longer warranties are more about marketing than anything else. If there is an actual defect in the material it will usually show up early in the life of the mattress but knowing the quality and durability of the materials in your mattress are a much more reliable way to assess the durability and useful life of a mattress than the length of a warranty. There is more about mattress warranties in post #174 here.
The most reliable way to assess the durability of a mattress or know whether it has any potential weak links is to make sure that you find out the type and quality of the all the materials inside it (see this article). There is also more detailed information about the many variables that can affect the durability and useful life of a mattress relative to each person in post #4 here that can be a much more reliable indicator about the durability and useful life of a mattress than the length of the warranty.
I would avoid coil counting as a way to determine the quality, durability, or value of a mattress because an innerspring isn’t normally the weak link in a mattress and the number of coils is only one of many factors that determines how an innerspring will feel and perform inside a specific mattress design and can be more misleading than helpful. There is more about the different types of innersprings in this article and in post #10 here but I would pay much more attention to your own careful and objective testing which will tell you how the innerspring performs in combination with the other layers and materials in a mattress and to the quality of the materials above the innerspring which is normally the weakest link in a mattress than I would to the number of coils in a mattress which by itself isn’t particularly meaningful.
There is also more about the most important parts of the “value” of a mattress purchase in post #13 here that can help you make more meaningful quality/value comparisons between mattresses.
Without all the information about all the materials and components in a mattress it’s really not possible to make any meaningful comments about it or identify whether it has any “weak links” in its design. If you can find out the specifics of all the layers and materials in any of the mattresses you are considering that are listed in the “mattress specifications you need to know” article that I linked and post them on the forum then I’d certainly be happy to make some comments about the quality/durability of the materials or the mattress as a whole.
Phoenix