Seeing the waveform allows you to chop samples visually...as well as with your ears. On the 500, you get a set of numbers and you basically have to use your ears to chop...it is the old school method. Is a visual waveform necessary? No. Is it nice to have? yes. I own both of these machines and they are both relatively easy to use.
That said, you can get a used MPC1000 for cheaper than an MPC500. If you do not need the portability, I'd say get the 1000.
Korg Volcas / 6 x TE POs / MicroBrute / EH Space Drum & Crash Pad