Definitely Lithium-Ion, most for due to the longevity of the batteries. My Makita 18V drill is going on nearly two years old and I use it daily at work. It has tons of cycles on the batteries (Li-ion). Actually, all my power tools are Li-Ion now at home and work.
'13 1SS/1LE-Inferno Orange-Borla SType-Katzins-RotoFab-Elite Engineering-Michelin AS3-IOM Interior Pack