The keys on the PA600 are not semi weighted and do not have aftertouch like the 900 does.

The PA600 can play MP3's but cannot record MP3s which the 900 does.

There are 2 players on the PA900 and 1 on the PA600.

There is a programmable knob on the PA900 and not on the PA600 and three programmable switches on the PA900 and 2 on the PA600.