AGMs are a bad choice for deep discharge use. They are the same as flooded batteries but with a fiberglass envelope around the plates, a catalyst to hopefully recover some water from oxygen and hydrogen gas, and the caps are glued shut. They cost more and don't last as long.
AGM is designed for shipping automobiles standing on their tails on rail cars, so the acid doesn't leak out.
There is not really any chemical reason they would charge faster. And they are less robust - overcharge once and they are ruined, while with a flooded battery you just put the water that boiled off back. Adding water once in 6 months seems such a low cost for a much better battery.
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