Frequency of use
If the tool will be used often and repeatedly, look for a high voltage (longer) battery life. If not, focus on what you want to accomplish with the tool and look at individual features like chuck or torque size.
Weight and ergonomics
Lithium-ion is more lightweight than NiCad batteries. Conversely, NiCads are known for stability and toughness. For frequent use, the Lithium-ion is more lightweight and therefore more ergonomic. NiCad is ideal for heavy duty work over continuous periods of use.
Charging time & battery strength
Lithium-ion and nickel cadmium – NiCad – both provide the same amount of power, but lithium-Ion uses the power more efficiently. Lithium-ion is the lighter, longer-lasting and more expensive choice; NiCad is the heavier duty, more consumable, and less expensive choice.
The chuck size. For ordinary around-the-house use, you don't need anything more than a 3/8" chuck size. Heavy load work should consider a 1/2" chuck size. For torque, the higher the torque of a cordless screwdriver drill, the better the twist capacity.