They are really different animals. The 4x has fantastic real-time controls, even down to details such as labeling on top where the pedals, mic, etc plug in, so you don't have to fumble or lift the kb to see them. The vocal harmonizer is o.k. in the SD7, but simply great in the 4X. I prefer many of the lead sounds on the Ketron. Styles on both are good, but I enjoy a lot of the Unplugged styles on 4X.
Screen resolution is better on the SD7, BUT it doesn't tilt.
I prefer the easy hookup for Korg EC5 pedal. The Ketron still has one of those old screw-in cables that printers used to have.
I prefer the standard power supply on the Korg, rather than outboard. Size of the Korg is much better and the layout is nearly perfect for live play.
Chord sequencer and midi-to-style converter on the Korg are nice.
I prefer the keytouch on the Ketron, but bear in mind that for me, lighter is better. Both are fine though.
The sliders are in a great position on the SD7, but there are more on the Korg and they are all programmable.
The Korg fits a 61 note stand and bag. SD7 is 76 note size.
Many more differences, some of advantage for each kb. As I said they are really different.
It would come down to personal preference.
Ketron has only AJ for support in US. However he is easier to reach than anyone at Korg, although Paulo monitors the Korg forum and is helpful. You can't pick up the phone and call him though.
I like both arrangers. Hope I'm still around when the successor to the 4X comes out in a year or so!