An AGM user here. I have a single 4D AGM. It gives about the same AHs as 2 6V in series. AHs are AHs no matter how you arrange them.
I like having 2 terminals to keep clean instead of 4. No rust or corrosion in my battery box area. My 4D has had 5 seasons so far. I expect at least another 3. For 6 months of the year my AGM sits, disconnected, with no battery minder. Nothing. In less than 1/2hr charging it is ready to go for the next season.
Yes it was expensive, but I like the peace of mind knowing that I have almost the best made & most reliable when I am out dry camping miles from any Walmart where the cheap 6V wet cells come from.