A very quick response for you....
The MPC 'Music Production Center' (MPC500) is a sampler - Can be used to record/edit/sequence user samples
The BPC 'Beat Production Center' (XR20) is a drum machine with a fixed sound-set (100 built-in drum kits - approx 700 samples), but NO user sampling. Basically, the XR20 is a sample play-back device with a fixed sound-set stored in ROM for playback only.
2 completely different beasts.
Check out the features on the Akai Pro website for more details on each device.
Hope that helps.
Akai MPC 5000