With known problems of the Li-ion batteries going out of control due to over-charging and such, my personal engineering decision would be to use the nickel-metal ones and have slightly less amp-hours, a little more weight, but be safe.
Gums - concur that sentiment - suspect that increase in weight may not be too much of a problem - but how much bigger would a nickel-metal battery be? Would they have the space? Looks pretty size limited from the pics I have seen
