There is also one other rarer expansion unit for the CD32.
This is called the CD32 Promodule is very similar to the original SX-1, but you place it under the CD32, rather than behind, and then connect a block connector between the two units. It has a built in floppy disk drive and the ability to add up to 8MB of fast ram and an FPU to the unit. It also adds similar ports to the SX-1, such as parallel, serial, RGB, external floppy, IDE and Keyboard.
I hope that helps you out. So I would definitely recommend the SX32 over the other two. The SX32 is also available as the SX32 MKII. This is just an updated version of the original and includes exactly the same features, except the PCB is smaller.