I will second the ABOVE...AGM is "Absorbed Glass Mat." Fiberglass mat impregnated with the electrolyte solution. The ones you want are heavy and designed for marine use. They can handle vibration and don't leak. They also last much longer. Well worth the extra cost, IMO. You do need to use a charger that will work for AGM, most modern chargers do.
I hate old style "wet" batteries where you always end up with acid leaking out and causing corrosion of terminals, burns on your skin and holes in your clothes. You are supposed to pop the tops to vent when you recharge and then you have flammable hydrogen gas to deal with. My Dad had a Porsche 914 once that folded in half from damage to the unibody frame from the leaking battery.
You can save money up front by buying inexpensive high quality marine batteries ($70 Interstate at Walmart) but in the long run cost ends up being about the same.