Both have strong points. If you want to use vocals through the keyboard, the 740 is better. It has harmonizer and fairly extensive microphone processing. The 5000 mic preamp is bad.
Also consider compatibiity with your primary keyboard. It's nice if they both use the same format as far as songs, styles, sounds, etc.
Also, the 740 is much lighter in weight.
The KN5000 has some really neat features, such as creating styles from songs, style conversion software and color display.
It all depends on your specific needs.