Hi Dekabal,
Welcome to the Mattress Forum!
Your initial impression is probably accurate, and a mattress that is softer on top is generally not the preferred comfort for most who sleep upon their stomach. There’s more in mattress firmness/comfort levels in post #2 here that speaks to mattress selection for different sleeping positions.
The Silver Lake Firm uses a different pocketed spring unit from the Silver Lake Plush that is listed to be firm (most likely less contouring than the spring unit in the Silver Lake Plush version), but they don’t list if there is any difference in the softness of the polyfoam in the quilt as compared to the Silver Lake Plush. The Highland is listed as a “medium” feel, and looks to use the same innerspring unit as the Silver Lake Plush, without the 2" latex layer.
Overall, the Highland should feel a bit firmer than the Silver Lake Plush, and the Silver Lake Firm should feel the firmest of this group. I would be sure to go visit your local showroom again and test out these items and see if there was enough difference in comfort between the Silver Lake Plush and the Highland to justify the change, or if you should switch to the Silver Lake Firm for prone (stomach) sleeping.
Phoenix