As been said there are a few more efx on the 600, but also you can change the timing of your efx (e.g. 1/2 beat, 3/4 beat, etc) at the push of a button whereas with the 500 you have to do it manually (per se) with the parameter knob.

There's certain other little things like you don't have control over the crossfader curve on the 500 as well.

And I don't know if this applies to all 500s / 600s, but on the ones that I've used the EQ on the 500s was much smoother/not so harsh, whereas the 600's EQ was almost like a kill switch when turned completely down.

All in all though, for the few extra quid it'll knock ya, you'd be better getting the 600 imo.