The NL121 plugs into the data logger and doesn't require an additional cable to work. The NL121 also doesn't take up much extra space. Because the NL121 has a faster interface to the data logger than the NL201 does, the NL121 communicates faster.
The NL201 has to be mounted next to the data logger and requires a cable to connect to it. Also, the NL201 requires some additional configuration so that it will work with the data logger. For example, choices have to be made regarding how the NL201 is connected to the data logger.