The other difference is that if you are using 3 of the channels like if you were playing with 3 turntables, on the 600 you can use the crossfader to cut between the channels whereas with the 500, you would have to turn the crossfader off to be able to manipulate all three channels. The crossfader on the 500 only works between 2 channels.

You also get more control of the effects with the 600 versus the 500.

If you were choosing between the two, I would recommend the 600. That is my opinion however.