Trying to avoid the whole of mire of what AGM Battery is best and just replying to the OP here, because I don't think anyone really answered the question.
To start, it realistically depends on what your requirements are. Basically you want some power for your accessories and want your engine to start in the morning.
Another consideration here. Do you plan on driving fast? All the folks I know w/ group 31s and run deserts end up getting a crack in their fender well because the weight of a group 31 isn't so good in that spot.
I have a Group 31 AGM in the front, I don't drive that fast over rough terrain, and I have it powering an EdgeStar FP430 Freezer, my Kenwood TM-V71A Radio, my 12v Electric Blanket, Drone Battery charger, a multi-port USB charger that powers 4 BioLite PowerLight Minis, my phone, my tablet, and a bluetooth speaker for dem beats.
My only requirements were a deep cycle that can handle my fridge and lights over a 12hr period. I'm usually driving during the day so I don't have problems w/ the battery running down. If i'm parked, solar on the roof will keep the battery charged. At night, the battery only needs to run my 20w electric blanket, 20w of lighting, and 60w intermittently for my freezer. I've done in place camping in the summer and the winter and it's been great over the past few years. TBH even the stock battery in my Tacoma was fine, but my 4runner needed a new battery so I moved it over and got an AGM for the truck.
Honestly, the cost of a Dual battery setup isn't expensive at all. You don't need a fancy battery tray if you don't want one. Many people use the stock battery tray and a different tie down. I hear zip ties are cheap. If you run a second battery, you just need to isolate your load to only the second battery and use a DC to DC charger which is like a $100 bucks. A 100w solar panel with a pwm charge controller is like $170. Inverter, if you really need one is like $20 bucks depending on size. Then just use whatever battery disconnects or blocks, or whatever to hook it up. Your starting battery will never run down. If you want to protect your house battery, install an LVD.