Here's a good article on AGMs. They need a different charger than an ordinary 12 V charger to make them last. The chargers I have used on AC batteries were expensive so I don't care for the batteries Though they are great in Airplanes and boats. Deep cycle marine batteries have a better clearance between the lead plates and the bottom of the battery. When a lead acid battery becomes discharged both electrodes undergo chemical changes to form lead sulphate and eventually the battery dies from buildup on the floor or bottom of the battery which causes it to touch the lead plates.
http://www.cmmonline.com/articles/231347-learning-about-agm-batteries