Great question. Beside the main differences in power and lifespan, here are a few of the things that set them apart once they are in your hands:
The SJ models are very simple, easy-to-use systems. They have status lights that verify the unit is working, tell you at what power level it is set, and whether you need to clean the cell or add salt to the pool. Because it is simple there is less to think about, it is particularly low maintenance (no acid is required to clean the cell), and it is also less expensive.
The RJ and RJ+ models are the flagship systems. They are more powerful and longer lasting. They have a digital screen and touch keypad, giving you more control and diagnostics. It displays the salinity level in real time, calculates the running average, displays the water temperature, and displays the electronic performance levels of the cell so you can see exactly how things are working. The power levels on the RJ models can be turned up and down in smaller increments. RJ models also synchronize more easily with variable-speed pumps if you plan to upgrade.
So the SJ gives you simplicity and a huge value, and the RJ gives you top of the line performance and control.