Thin plates are the storage parts. It's still the best instrument to find out if battery is charged. The more discharged the battery, the more stratification will occur. But need more of them and more maintenance. They're 6 volts, so need two to get 12 volts.
Issue is needs more connections, some loss in that, plus more chance of corrosion.
But need to add water more often, and equalize more often. AGM is 2-3 times more expensive. Problem is overcharging causes leakage, so they can't be charged at more than around 14.2 volts. Charge slower and longer. AGM works better on smaller batteries than larager batteries.