Hi Hunter3668 and welcome to the Mattress Underground 
I’m new to the whole mattress universe to be honest. Just recently had a newborn who sleeps in the bed with us. Wife doesn’t like our bed says her back hurts but deals with it because its our only bed.
Congratulations on your new baby! Sorry to hear your mattress is no longer holding up…
I’m sure you and your wife need the little sleep the baby allows! Since you are just entering the ‘whole mattress universe’, let’s start with the basics. As you may know, any mattress the two of you choose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health issues) - thank you for providing those - and PPP (Posture & alignment, Pressure relief, and Personal preferences). A few resources to read are the Mattress Shopping Tutorial to see how to determine a great mattress manufacturer or showroom, and the Mattress Specifications You Need To Know which can be compared to the Mattress Durability Guidelines to see the durability of specific materials over time.
Here is the bed info Queen mattress brand name Sapphire
I took a look at the Sapphire Sleep Thermic Cool Phase Hybrid which you have now; this is a 14" mattress with 2" of gel memory foam of undisclosed density, over 1" of latex of unknown ILD, with a 6" wrapped innerspring coil support and 3" base foam of unknown density. Your wifes’ back pain may be in part dire to to top gel foam comfort layer prematurely breaking down, b’bottoming out’ on the thin latex layer and resting on the support layer. Unfortunately it is quite common for the ‘big brands’ to be vague in their specifications…this can hide excessive amounts of inexpensive foam which can prematurely wear out causing ‘Valleys’ and ‘ridges’. If you both like memory foam, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. Any mattress which lacks information on the thickness, layers, and density raises ‘alarm bells’ for me, and personally I avoid those as it’s impossible to assess their support long term durability without this info.
Wife: -Back sleeper -Prefer medium to a little sturdy. -Weigh 140lbs -Height 5ft 7in
Me: -Side sleeper -Prefer medium-soft, plush, etc. -Weigh 160lbs -Height 5ft 7in
As you are a higher BMI ,and prefer a softer feel while your wife likes a medium firm, you may want to consider a split king, so you both can customize the comfort layers to your preferences…though with a higher BMI, too soft of a top comfort layer will allow you to sink and bottom out on the layer(s) below, causing muscle and joint aches and pains and even alignment issues.
As news parents, I’m sure you have little free time, but if possible, taking a trip to a local showroom just to try out some different mattress types, like latex, hybrids, microcoil layer, etc to see how they feel. You can thereby make a ‘short list’ of mattress types and can then consider the best solution for your needs, preferences and budget.
You may want to look at the offerings of the Trusted Members of the site as these have all been vetted for quality, transparency in materials and construction, great customer support and generous return and exchange policies. If you provide them with a little information, like Stats, PPP and mattress history, they can help you to find a great sleep system which will last for many years. If you see any specific mattresses you are interested in, feel free to post and we can possibly comment on how they may work for your needs.
~ Basilio