Hi mloehlein,
Welcome to the Mattress Forum! 
Passing on the Serta is just as well, as the major brands such as Sealy/Stearns & Foster, Simmons, and Serta all tend to use lower quality and less durable materials in their mattresses than most of their smaller competitors that will tend to soften or break down prematurely relative to the price you pay which is why I would generally suggest avoiding all of them completely (along with the major retailers that focus on them as well) regardless of how they may feel in a showroom along with any mattress where you aren’t able to find out the type and quality/durability of the materials inside it (see the guidelines here along with post #3 here and post #12 here and post #404 here).
There is more information in post #9 here about the different ways that one mattress can “match” or “approximate” another one. Every layer and component in a mattress (including the cover and any quilting materials) will affect the feel and performance of every other layer and component and the mattress “as a whole” so unless you are able to find another mattress that uses exactly the same type of materials, components, cover and quilting, layer thicknesses, layer firmnesses, and overall design (which would be fairly unlikely) then there really isn’t a reliable way to match one mattress to another one in terms of “comfort”, firmness, and PPP (Posture and alignment, Pressure relief, and your Personal preferences) based on the specifications of the mattresses (even assuming that you can find out all the specifications you would need for both mattresses you are comparing in the first place).
You could start by looking online and use the experience and expertise of the members listed in post #21 here who are all very experienced and knowledgeable and specialize in providing the type of help and guidance on the phone that can help you make good choices. There are a wide range of memory foam options included in the choices there and I believe that all of them compete well with the best in the industry in terms of their quality, value, service, and transparency.
In addition to the online members of the site listed in the previous membership link … post #12 here has some of the better online memory foam options for those who are looking for a memory foam mattress. For a more restricted budget post #4 here and the posts it links to also include many of the better lower budget online options I’m aware of as well.
A good online retailer or manufacturer will generally make suggestions that they honestly believe have the best chance of success based on the information you provide them when you talk to them on the phone because this is in both your own and their best interests, and you can convey to them what you’ve tried locally and liked, but at the end of the day the only way to know for certain whether any specific mattress is a good match for you in terms of comfort, firmness, and PPP will be based on your own careful testing and/or your own personal experience so if you can’t test a specific mattress in person then the options you have available after a purchase to either exchange the mattress or individual layers or components or return the mattress for a refund (and any costs involved) would generally become a more important part of your personal value equation just in case a mattress you purchase doesn’t turn out as well as you hoped for.
I hope this is a good start for you and helps to point you in a proper direction. Keep me updated on your progress!
Phoenix