There's one scenario where an AGM comes in handy for these trucks, IMO, and that's if you seldom drive the vehicle. That's because AGM batteries discharge more slowly than the conventional flooded lead-acid (FLA) type.
They also feature less resistance when cranking (found this out just today, as it turns out), and they also can be regularly discharged to about 30% without harm (vs. 50% for an FLA). Finally, since they're a true 'sealed' battery, they can be mounted in any orientation (even upside down!), which is nice if you want to do a trunk relocation in a car, or install in an RV, etc. And if you order from someone online, that's a benefit when it comes to having it shipped, as well.
Where compatibility issues rise with our trucks is in the charging algorithms... AGMs are 'supposed' to be charged at a slightly lower voltage / rate than an FLA (think BBQ... 'low and slow').
I said 'supposed', because our trucks can't really discern between the two, so they charge the battery as if it were an FLA (as high as 15.x volts, depending on how discharged it is.) Our charging systems do have *some* intelligence built-in, but not to handle AGM batteries 'properly'.
Is that bad? Jury's out, as far as I know -- I bought one a year or two ago, and haven't had issues with it, but I'll be the first to say -- that's not a long time. The reason I bought it is because my Envoy does sit for fairly long periods (two weeks or more; especially this past year). I've not noticed any audible difference in cranking, after it sits (but it doesn't really crank any differently than the 'old school' FLAs did, either).
So... if you're driving the vehicle even semi-regularly, a FLA would be just fine. Your pick of what brand. I used to get the WalMart batteries because if I were out camping, and one went south, there's a WM just about everywhere, when I'm away from home, and they carry batteries in just about every WM (even the ones without auto centers).
Unfortunately, I had two of their 'Maxx' batteries go bad (one in the Envoy, one in a Sierra), so I swore them off going forward. I have one left (as a warranty replacement), and I keep a Bluetooth device attached to it that tells me its state of charge (SoC) at least once per day - when it gets close to 11.9 V (the 50% mark), I break out the charger. But when it goes, I'll give serious consideration to another AGM (especially if I can find one on sale, like the current one in the 'Voy.) What I've learned (?) over time, and using that monitoring device, is that the WM batteries I'd been buying have a faster discharge rate than some other brands. They're made by Johnson Controls, who makes batteries for many other labels, so I don't know exactly why the WM batteries don't hold charge as well (except maybe there's some sort of testing of lead plates, and the WM contract allows them to use the 'marginal' ones.)
Anywho...as far as what type to get... since you have the V8, like I do, something in the 800 CCA range is good (the OEM was about 650 CCA or so, as I recall). It's a Group 78 -- but if you don't like side-terminal batteries, you can get a Group 34, which is the exact same battery, except that it's a top post. You can get a set of terminal adapters that convert the wires in your Denali to top post, and use a Grp 34 just fine, if you prefer.
There are some batteries that even have *both* terminal types (they'll be listed as a '78/34') -- if you get one of these, I'd keep the terminal protectors on the set you don't use, to protect against accidentally shorting the battery out, if it comes into contact with something it shouldn't. Other than that, they're just fine to use.
When you get your new battery, try to get one as 'fresh' as possible -- there's usually a sticker or tag that tells the manufacturing date. Less than six months old from 'today' is good (and about the most time from 'new' to accept one, unless the store can assure you that they charge their stock at least once a month (the 'Batteries and Bulbs' chain does do this; I've seen it in their stores. WM... well, they don't, as you'd expect)).
Less than 3mos from mfg date to installation is even better. And I do top off any battery I buy with a charger, before installing it in a vehicle. That way, I know it started out as optimally as I could get it, and my alternator doesn't kill itself trying to revive one that might be almost dead.
There are a couple of other battery types - Gel cell, and Lithium. The Gel cells *do* require specialized charging, and the Lithium are really expensive, especially for this application. So I'd stick with the FLA or AGM; spend the difference on a decent charger (e.g.; one that has an AGM setting, if you do decide on that format).