Hi Xiong,
Unfortunately there are too many variables involved in the type and design of an innerspring (see this article and post #10 here) much less the many possible combinations of foam layers on top of the springs to be able to provide any meaningful answer to your question based only on a single spec (the gauge of the wire). The only thing that I can say with any conviction is that a 13.5 gauge single coil that is in the same type of innerspring system, has the same shape and design, number of turns, diameter, and height will be firmer than a 14.5 gauge single coil that is otherwise identical. The most reliable way to know the difference between any two mattresses “as a whole” would be your own personal testing or experience. If you aren’t able to test a mattress in person before you buy it then the “next most reliable” way to find out the differences between two mattresses would be based on a more detailed conversation with a knowledgeable manufacturer or retailer who will know more about their mattresses and how they “feel” than anyone else (see mattress firmness/comfort levels in post #2 here).
I can certainly provide information about the quality of different materials and help you with “how” to choose and make more meaningful comparisons between mattresses (and how to find reliable information) but unfortunately I’m not the best source to provide meaningful information about the comfort/support of a mattress that I have no personal experience with and have never slept on and even if I had, my own personal sleeping experience and subjective perceptions of any mattress may be very different from yours.
Phoenix