The MPK is a MIDI controller only; the MPC is an entire workstation. That means that the MPK alone will not make any sound. You need to plug it into a computer or other sequencer in order for it to control the device it is plugged into and make sound. The MPC, on the other hand, has all of the sounds built in. It can stand on it's own and doesn't need any external gear to make music.
Both devices can achieve the same end result, it's just which way you choose to go about it that determines which device would be best for you. If you plan on doing lots of live shows, the MPC would probably be more helpful as you wouldn't need to bring along a computer to the stage. If you plan on using this device in the studio only, you could get by with the lower priced MPK.