Depending on what the application. Like JimB said, some PLC's come as a contained 'One box' or as we call it 'Black Box' that offers some 8 DI (Digital Input) and 4 DO (Digital Output). These small units seldom allow for any analogue signals though.
Most PIC's can handle the same IO but unlike the PLC where the IO is 'Wire and Go', with the PIC you will need circuitry to connect the IO to the PIC. With some PLC's you get the programmer tool software with the purchase of the unit. (Most case for the small units it might be Ladder). With PIC you have all the different packages available, some for free (i.e. MPLab), and the others (like C compilers etc.) will cost a pretty penny.
Connecting a Keypad to a PLC is most often a waist of IO. PLC's make use of dedicated protocols like Modbus (antiquated by now) PROFIBUS and many more to connect peripherals like Display panels, industrial keypads, smart IO and many more.
The main difference in short... THE COST.