If you value being able to produce beats on the go anywhere and everywhere, go with the MPC500. Can be plugged in and can run on batteries. It is a very powerful tool.
The big difference for me was that the MPC 500 does not "Phrase Sample". The 1000 does and the 500 supports it but if the 500 phrase sampled, you wouldn't need any of the other MPC units. Phrase sampling the the main dran to the MPC in my opinion. It speeds up the process of chopping samples.
You can find videos of how people use the MPC to Phrase sample and you'll see what I am talking about.