S7-200
The S7-200 is referred to as a micro PLC because of its small size. The S7-200 has a brick design which means that the power supply and I/O are on-board. The S7-200 can be used on smaller, stand-alone applications such as elevators, car washes, or mixing machines. It can also be used on more complex industrial applications such as bottling and packaging machines.
S7-300
The S7-300 is used in more complex applications that support a greater number of I/O points. S7-300 is modular and expandable. The power supply and I/O consist of separate modules connected to the CPU. It is chosen often by the industries that have future expansion plans.