I do not know enough about the S7xx to offer detailed advice on its sound editing capabilities but do be aware that there is now the new SX models which are now available and hence must cast doubt on how long the S models will continue. Videos on the SX models are on the Yamaha website
Please watch this comparison which was made by my local Music Store who are main agents for Korg, Yamaha, Roland and Nord.
Personally to me the Korg blows the Yamaha out of the water, and whilst it is about the next models up the sounds of the 700 are very similar.
The 700 is worth the price difference imho.
As Michael suggests, research on the models and add in a visit to a Music Store to test out the models will help you greatly.
I have had my PA700 for three years and love it but not trade it in for anything other than another Korg.
One huge benefit of Korg is product support which does just that plus both Korg UK and Korg USA hold monthly live webinar and Q&A sessions where the demo the features and the custom tweaks that can be applied.
Finally if you can provide more information on what precisely you are seeking then perhaps more specific advice can be given.