Hi rhemy1, and welcome to The Mattress Underground 
Happy to have you here with us.
As you likely know, a mattress is unique to the sleepers using it, based on their stats (height, BMI, sleeping position(s) and any underlying health issues, and the sleepers’ PPP - Posture and alignment, Pressure relief, and Personal preferences. As I am not sure what you’ve read since finding the Mattress Underground website it’s worth mentioning that when searching for a new mattress, it’s always advisable to first review the
mattress specifications you need to know, as well as the mattress durability guidelines, to know what to expect in terms of mattress longevity and how long you can reasonably expect to sleep well without the
mattress losing its initial comfort feel and supportive properties.
Also, you can see ourmattress shopping tutorial to gain some insight on what to know when searching for a mattress.
You do not provide any of your stats, but the two mattresses you mention are quite different. The Jordon’s Akashi Chill Touch has two one-inch latex layers of 24 and 28 ILD (indentation load deflection) which are good quality materials and will last for a long time which they consider a ‘medium-firm comfort level’ on their scale. They do provide the density of the polyfoam layers as 1.8lbs/cuft which is considered lower density for higher weight individuals. The Serta iComfort limited edition mattress, on the other hand, is comprised of layers of gel memory foam of unknown composition, density, or thickness which is common among the larger retailers, and the lack of transparency in the details of their components can mean these are possibly inexpensive, low quality foams… higher thicknesses of low density foams can be a ‘red flag’ in that they will likely ‘break down’ prematurely which can cause weak spots or divots on the mattress, that can cause sleepers discomfort, spinal and joint misalignments.
(10")Serta iComfort limited edition
Cool touch cover
gel foam unknown density or thickness (Fuze)
gel memory foam unknown density or thickness (Fuze)
Comfort level plush by their scale … As you are looking for something more firm this may not be the best of the two options in terms of comfort.
You can see in Post #12 here where Phoenix discusses some of the major mattress retailers. It’s good to be informed when making an important purchase like a new mattress…all this being said, as it sounds like you have tried these two mattresses in person, it comes down first to how durable each mattress will be and then to how comfortable you are on each of them when testing.
I can’t say anything about which of the 2 mattresses you are considering would fit you better in terms of comfort and support but in terms of durability the Jordon’s Akashi is a much better option as you don’t really know much about the quality of the foams in Serta.
Since you say you may ‘order from a different supplier online’ you might want to consider the offerings of theTrusted members of the site. Most of these offer online sales, and are excellent at providing consumers with mattress that are right for them, with quality construction, transparency in materials used and great customer support; If you provide some relevant information on your stats and preferences, and experiences with mattresses you have tried out, they can provide a durable comfortable solution for your sleep system needs.
Basilio