In general, if the part has "08" (for example, RS08, HCS08, HC08) in it, the MCU is 8-bit. If the part has a "12" in it, the MCU is 16-bit. If the embedded application will manage simple control functions, then you often use an 8-bit part. If the embedded application will require data processing (generating packets for a network connection, sampling an ADC and computing an average) then a 16-bit part, which has more computing power, might be used.
That is an oversimplified explanation, of course, but is the essence of why there are the two different MCU types.
---Tom