Wow that was more response than a guy could ask for! Answered my Q, plus more insight. Thanks guys.
BTW, what is an AGM battery?, and what is a gel battery?
To start with, they are about twice as expensive as an ordinary battery.
An "ordinary" battery is called a wet cell - the electrloyte is a fluid (sulfuric acid). As a fluid, it has a greater tendancy to lose volume, so you have to add water to make up for the lost volume. A completely sealed wet cell can be maintenance free.
An AGM (absorbed glass mat) has a different construction where the electrolyte is suspended between the plates in a fiberglass mat-like material.
A gel cell has silicates added to the electrolyte to turn it into a gel.
Both have less tendancy to lose electrolyte and to self-discharge.
Both are maintenance free batteries. Both may require special charging to avoid overcharging voltages.
The AGM battery is kinda the gold standard for motorcycles, especially one like the KLR where the battery gets cooked and getting the thing out to maintain it is a bit of a pain.