You will realize these two batteries are designed in a much similar way on taking apart any of it. They both have individual battery cells which are arranged in a group of 5 wired in a series. This is done to ensure that the battery has a significantly large number of amp hours and good capacity in terms of watt-hours. So there is no real difference between the two battery types of drill. But here the question arrives 18V Vs 20V drill - which one to choose?
Torque - While purchasing most important factors comes into mind, what’s it maximum torque? If you are looking for maximum torque, then you should go for 18v as compared to 20v drill offer better torque. The 18V drill provides higher torque which is the better result when you drilling through higher surface.
Size and weight - A 20V drill is quite heavy which may cause a lot of difficulties while working as compared to 18v drill which is lightweight. No matter whatever you purchase but the ultimate power you get at the end of the process is the same.