Another type of VRLA sealed lead acid battery is an absorbed-glass-mat (AGM), maintenance-free type. My motorcycle has one. It has compressed glass mats between the plates to absorb and hold the acid so it stays between the plates and will not spill. The one on my cycle is a dry charge type battery so you initially fill it with acid from the supplied vials. After that you attach the caps which snap on flush and can't be readily removed.

I think the advantage of an AGM battery over a gel-cell is that the AGM can provide higher starting currents (has lower internal resistance) for a given size.

 

AGM type batteries are the best I've used. They tend to be at the upper end of the price range, maybe a bit beyond. Optima yellow tops for deep cycle and Red Tops for starter batteries. They don't drain much after sitting for a while, and charge fast. Very long lifetime as well.

 

http://motolite.com/battery.php?id=21