For all who are interested in Total Cost of Ownership (TCO) of various types of batteries, see this link where someone in a boating forum did the comparison math (see post #11 from jallum):
LiFePO4 Batteries - Okay, Tear Me Apart ;-) - Cruisers & Sailing Forums
Assuming he did the research and math right, we all should be considering either AGM or LiFePO4 batteries. Of course, since these are 24 volt batteries, you need to either extrapolate from these comparisons or do the same research and math yourself for 12 volt batteries.
The LiFePO4 (Lithium Iron Phosphate) batteries are very interesting, since they are much, much lighter and smaller, but have characteristics much like AGM batteries although the LiFePO4 batteries permit about 7.5 times as many cycles as AGM batteries.
For info about LiFePO4 batteries, see these links:
M2 Power: Lithium-iron-phosphate (LiFePO4) batteries, chargers and battery management systems (BMS)
Lithium iron phosphate battery - Wikipedia, the free encyclopedia
What do you think?