it all depends on what you want to achive.
I have the Sx-1 expansion for my CD32, and it's good for getting the most oout of the machine - it has extra fastram and an HD, and a floppy port and Keyboard port.
This means i can pretty much treat it like an A1200, and even run WHDload installed games on it.
The unit itself however, is bulky and disconnects from the machine a lot. An extra keybaord also takes up an annoying amount of space. These sort of things are exactly why i prefer my A1200. (nice cosy, all in one unit).
The SX32 expansion is better, and also comes with a better processor. This one slides "inside" the Cd32 mostly, rather than hanging off the back. This is by far the most desirably, and therefore expensive option.
The floppy expansion, as far as i know, doesnt given anyting else does it? No fast ram, no HD options. If you just want to play a few games that are CD32 compatible, (you may find a fair number of your favourites are not CD32 compatible on disk, although you can use a program called "Relokick" to emulate kicstart 1.3 and improve compatibilty a bit) and dont require a keyboard, thats fine, but a bit of fastram and an HD goes a lot further in you choices of games to play on the CD32 than a floppy drive does.
Hope that helps slightly