The Korg is a much better performance instrument because:
- it is smaller
- it has tons of hardware buttons for practical live performance without the need to navigate the screen
- it has many great performance features that Yamaha doesn't have (Kaoss pad, chord cycle, ability to trigger 2/4 in a 4/4 style. etc)
- it has a more responsive screen
- sounds and style are HIGHLY editable
- you can choose 71 or 76 keys

The Yamaha is a much better studio instrument and has other advantages:
- So many of the sounds are so much better
- It is lighter than the Korg
- Very nice GUI