Hi Sonya,
The scrubber dryer is normally used for the cleaning of hard floor surfaces, mainly large area's, although you can get smaller versions for doing smaller area's. They clean the floor whilst extracting/drying at the same time making them safe to walk on almost immediately. This is not really an essential piece of kit unless you have the volume of work where it would be required. A good flat mopping unit with microfibre heads would be more versatile.
The rotary machines are extremely versatile pieces of equipment, they can be used for cleaning, stripping and polishing hard floors, low moisture cleaning of carpets, shampooing etc. If you are going to be stripping and sealing hard floors then one of these is a must have, you can pick them up quite cheap second hand. If you do want to get into cleaning hard floors I would advise going on a training course to familiarise yourself with equipment, chemicals & different floor surfaces.
Derek