Here's my take from watching this forum and others.....
The ONIX has the scroll back capability, the screen can be split 4 ways and you can customize your own favorite views can be set up, you can operate it in touch mode or use a combination of a joystick and vernier dial. (You can adjust the sensitivity up and down by turning the dial.)
You can turn the ping of the DI element on and off. The SI ranges does not seem to be hurt by the dedicated DI crystal like it is on Structure Scan. (There is a little washboard effect like the Lowrance units have, but is very minimal and can be totally eliminated by tuning off the DI ping.) If you split displays, you can 2finger touch the screen on any one of the splits and it will go to that view full screen.....Another 2 finger touch and it goes back to the split view.
The SI and DI can be ran at different frequencies.....from the images I have seen it works really good when the DI is 800Khz and the SI is 455 Khz.
Probably a few more things I'm not remembering right now.

The DI images I have seen look better than any I have seen from any other unit. They are nothing short of amazing.......PM me and I will send you a link to some images.

If you are ever going to get a 360....the ONIX images for the 360 have better clarity.

It is more $ than the 1199, but a LOT better unit. Looks like the ONIX is the new king of the hill. I wouldn't be buying the 1199 ......it would be the ONIX for me.

From Greg on another forum: The ONIX platform has more processing capability than the 1199ci HD Si unit. It also has more connectivity than the 1199. The transducer has DI sonar (not Di sonar from Si sonar). More mapping card options plus SmartStrike capability. Not just touch screen but gesturing as well. Display size and pixel count differences. Digital Signal Processing for the sonar plus CHIRP. Built-in NMEA 2000 connectivity (no more AS-ETH-NMEA2K accessory needed). Customizable Views.