Ok, I had a chance to play on a piano like the 620, but in earlier version (502 or something like that) and I decide that I want a 7 octaves organ.

So it's or the 520 or the 620.
I managed to find only two difference: 1.better piano sound, 2.hard(piano-like) keys instead of half-hard.
Are there more differences between the two? because the price difference is huge!