Korg Arrangers come as standard with a fairly neutral setup. Good strings, brass, organs, wind and others, weak acoustic pianos. They do benefit from a good Eq setup and can be very easily have individual sounds customised. A strength is that the layering can produce great combinations and if one knows what to do by way of customising the oscillators to max out on combined voices the resultant sound of say strings and piano can be saved as a single sound for later use as needed.
Yamaha, well there is not much I can say about the 770 other than I played one back to back with a 700 and it was no contest the 700 beat the 770 in every way except certain piano sounds but even then the sound was too over bright and shrill for my own preferences.
The only experience I have of the 600 is playing one a while ago that one of the guys I jam with brings to the group but I have not been there for many months.
So it really depends on where you are coming from and your own preferences.
I cannot comment on the editing capabilities of either all I can say is the editing capabilities on my 700 is excellent.
My advice would be to physically check out all models in your price range by playing them, then save up more cash and buy a PA700.
The link below is the first from Korg’s video manual series (there are 7 in total) which will give you a grounding on each function