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.
2013 1SS/1LE Inferno Orange - Borla - Katzkins - Eibach Springs