So as long as you get a matched set of batteries (same size, rating, and if possible same production date) then it doesn't matter.
If you use the search function at the top left of the page and search this forum for batteries, you will come up with a lot of threads, as this is a commonly asked question.
Some people say only use the 6T series because that's what the military uses.
Other people will say go buy the cheapest ones from walmart that fit.
I'm in the middle. I am not a purist, and the only reason the military uses the 6T series is because they have other trucks that use it, so it simplifies inventory if they all use the same battery.
Group 24 batteries fit just fine and have enough cranking power (the original 6T was around 660 CCA, so anything with that or more is good) and the flat top/maintenance free version will work with the original hold down bracket.
AGM type batteries will handle vibration much better than flooded lead acid batteries will, so if you're doing much off roading they are a better choice.
The only real advantage to running 6TAGM batteries is the reserve capacity is big, so you can run electronics for longer with the truck not running. If the truck is running, then it doesn't really matter how much you run the electronics, as long as you aren't exceeding the output of the alternator.
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Personally I'm running a set of AGM group 24.
X2Power group 24 AGM replacement battery designed for low maintenance and dependable engine starting power. Designed for Volvo, Pontiac, Subaru, Dodge and more
www.batteriesplus.com