The "harder" keys on the 620 are more representative of a "real" piano. If you are not a piano player you may well find them uncomfortable to play for long periods until you build up some strength in your arms. The mechanism required for this type of keybed is more complex and heavier than the lighter "synthesiser" type keys; this is the main reason for the cost difference between the 520 and 620. The 620 also needs a stronger case to handle the extra weight of the keybed.

Regarding the sound, I always thought the 520 and 620 were identical electronically and had the exactly the same sounds and facilities.