Those Voltages are the capacity of the Rechargeable battery in each Cordless drill. If a Cordless drill is marked 9.6V, it means that its battery has a max. power of 9.6V and its motor is rated to run at only 9.6Vdc only. That also means a small size motor and a small battery pack, usually used for small time hobbyist, student and school project.
A Cordless Drill that is marked 12V or higher, is usually used by a professional for a heavy duty work. This drill have a bigger size drill (physical body to accommodate the big motor inside its housing), motor and battery pack that can run for a very long hours compared to a small drill with low voltage. Of course, because it has a big size motor then only high voltage (12V or 18V) can run them coming from a big battery pack.
Early cordless drills used interchangeable 7.2 V battery packs, and over the years available battery voltages have increased, with 18 V drills being most common, and higher voltage drills, such as 24V, 28V, and 36V, are also made .This allows these tools to produce as much "torque" comparable to some Mains-AC powered drills.
Its better to choose the high Voltage rated Cordless Drill if you are going to use it in heavy duty work.
Electronic Technician for 20 years