The 500 and 800 use different tone generators. Check it out. I could swear I hear a big difference between the two when A/B side by side in store. I just don't believe the 500 has the power (but it could be all my imagination).

There has to a be a big difference between the two, just based on the pricing structure.

You will never need another keyboard with the 800, but the 500 will grow old fast (IMO).