@Tribblerun To answer your specific questions, a deep cycle/marine battery is not normally used as a starting battery, which explains why you see it is not listed as compatible with your truck. As the name suggests, they are meant for a deep, but slow, power draw and recharge, which is typically a fairly low-current draw/charge on the battery. (think small trolling motor on a fishing boat). You can see that the 550CCA (
Cold
Cranking
Amps) is low relative to the normal starter batteries that are rated for 750+ CCA's. The likely reason it failed is due to the rapid and heavy electric current draw that happens when you start the truck, which that battery is not at all designed for. A normal starting battery is designed for short bursts of heavy current draw, which is why a similar/same size starting battery can be rated for much higher CCA's.
As for "flooded, non AGM", that simply means that battery is flooded with battery acid, also known as electrolyte. This type of battery is commonly called FLA (Flooded Lead Acid). These car batteries have been around forever. You may recall your dad saying to never leave such a battery on a concrete floor, which was correct many years ago but is no longer true.
AGM stands for Absorptive Glass Mat which is, as the name implies, a material designed to absorb and hold the liquid electrolyte which provides a few significant benefits, not the least of which is the electrolyte is no longer spillable. AGM batteries typically cost a bit more and usually last a bit longer than the FLA batteries.
To answer your question about which to buy...either FLA or AGM will work just fine. As several smart folks suggested, it is really best to replace both at the same time (sorry for the hit to the budget!) but if the working battery is also a deep cycle style then it will fail very soon anyway so best to avoid that problem if you can. My suggestion would be to try to find a sale on the AGM where the price is same or close to the FLA, and most auto parts stores or Costco/Sam's Club will install for free.
Good luck and let us know if you have any more questions.