On taking apart any of these two batteries you will realize that they are designed in much the same way. They both have individual battery cells that are arranged in a group of 5 wired in a series. Every group of 5 cells is connected through a wire in a parallel arrangement. This is done to ensure that the battery has a significantly large number of amp hours. It is also done to guarantee that the battery has a good capacity in terms of watt hours.
A deeper look at these cells reveals that each one has two different voltage ratings namely nominal and maximum. Every one of the cells in a 18v or 20v battery has a nominal voltage rating of 3.6 volts which translates to 18 volts nominal when put together. Every one of the cells in an 18v or 20v battery has maximum rating of 4 volts which translates to maximum 20 volts when put together. In essence the manufacturers of the 18v battery makes use of the nominal rating while the manufacturers of the 20v max battery make use of the maximum rating. This is basically the main difference between these two products.
Having noted the above it is clear that both of these batteries produce the same amount of power. The only difference is in the way they are advertised or labeled with regards to cell ratings. Another significant difference is that 20v max batteries are common in the United states while 18v batteries are sold outside of the United states. However, a person using 18v batteries outside of the US is getting the same results as one that is using a 20v max battery within the country.
It is equally important to note that there are tools designed to work with 18v batteries while there are also a group of tools that are designed to work with 20v max batteries. This can present yet another argument with a number of people preferring to go for the 20v max tool because it sounds more powerful. The information below should help you choose the right tool with regards to drills.