Most people say go with the 6T from NAPA if being cheap, or spend the big bucks on the 6TAGM.
Reality is, as long as the CCA is above what the 6T is designated at, and has a decently large reserve capacity, you can use pretty much anything that will fit in the truck.
Military likes huge reserve capacities because that way you can run radios and other equipment for longer periods of time and still start the truck, but unless you're a radio junkie or have a bunch of stuff hooked up like large stereos or inverters or stuff like that you want to use with the truck turned off, then the reserve capacity isn't as important.
Some things to consider though, NAPA sell the 6T battery for about $240, plus a $36 core charge, so about $275. Total cost about $500 with cores, $550 without (rounded numbers, add tax and stuff).
They are a decent battery, and meet the military specs. Only issue is they are a specialty battery, so only have a 6 month warranty. That's stupidly short for a battery these days.
The 6TAGM go for around $600. So $1200 for the pair. They come with a 3 year full replacement warranty (the ones I looked up do at least). That is a lot of money for a couple of batteries that you aren't really going to fully utilize unless you are running all sorts of additional stuff in the truck. Nice thing though is unlike traditional lead acid starting batteries, you can drain them down to 20% capacity (80% Depth Of Discharge it's called) hundreds of times without harming the battery. the flooded lead acids don't really like going below about 80% charge (20% DOD) or you'll shorten their expected lifespan.
Personally I went with Group 24 batteries. Batteries Plus sells some AGM batteries by NorthStar (one of the top battery manufacturers in the U.S.) that have about 2/3 of the reserve capacity of the 6T series, rated Cold Cranking Amps that are higher than the 6T but lower than the 6TAGM, cost half as much as the 6TAGM but have a 5 year full replacement warranty. The 24 series fits into the battery tray with no modification and held down by the battery holder without needing to use anything to space it up like the group 65s. You can put blocks around it to keep them from sliding if you want, but I haven't had any issues with them at all.
This page has some good info about batteries.
Everything you ever wanted to know about the batteries in a military HMMWV. What batteries to buy, how to keep them charged, how to secure them in the vehicle, etc.
gear-report.com
If you haven't seen it yet, there is a lot of good info at the site other than just the batteries.
Humvee Info – First Things Every HMMWV or Hummer Owner Needs to Know If you are new to HMMWVs or just haven't ever really gotten formally educated, here are some of the most important things to get you up to speed. The list below also makes a great pre-purchase checklist for anyone looking to buy
gear-report.com
