Buy the 1000, it is many times more versatile than the 500. If the price is the same it's a no brainer really. (Full blown smaller MPC vs. a cut down portable one) The 1000 also has an aftermarket OS you can purchase which even has a free version.
http://www7a.biglobe.ne.jp/~mpc1000/Larger LCD with more info, 16 pads rather than 12, the list goes on and all you really lose is battery powered operation.
Avoid a red and blue 1000 if the pads/sensors have not been upgraded. (Grab a corner pad and try to wiggle it back and forth. Look for the other pads to move slightly as well. If they do it's upgrades because the upgraded pads are are a uniform sheet of rubber).
I've bought 5 MPC's used and had no issues, although you should at least be ready to open the case (not difficult) to do maintenance, replace a broken button or just blow it out with an air can.
Read up on them here there's tons of info on either machine.