MK Battery appears to have very good prices on AGM-design batteries. The 8AGC2 (6v, 220amp, golf cart) and group 27 batteries are about $100 cheaper than AGM batteries from Lifeline or others.
I know that AGM batteries are never the budget alternative, but I have a good reason for wanting them. The battery location on my boat (the previous owners figured it out) is close to ideal in being out of the way, balancing out the boat (bow to stern and port to starboard) nicely, but has the downside of being so tight that battery boxes don't fit. The 8 year old lead acid batteries are on their last legs, and I feel like AGM would be a safer choice given the lack of battery boxes. My solar and shore chargers have safe charging patterns for AGM, but I'll need to replace my alternator regulator.
Do the advantages of 6v batteries (namely having thicker lead plates) transfer to AGM? The GC batteries have slightly higher capacity, but it is unlikely to matter for my low loads. My normal hanging around on anchor consumption is well under 2 amps.