The screen resolution is about the same. The ONIX has a higher quality screen.
Check the specs for each on the Humminbird website.
There are two ONIX versions, one is CT cross-touch ( touch screen and buttons) and the other is NT (just buttons). The NT version is about $500 less than the CT.
There are no touch screen Helix models.
The Helix 10 has a 2D, a DI, and a SI version.
The ONIX 10 has a SI version and a 2D version.
The Helix has pre-set views and the ONIX has customizable views.
The ONIX does CHIRP and the Helix 10 does not.
The Helix SI model generates the DI view by blending the two SI pulses.
The ONIX SI model has a dedicated DI crystal in the SI transducer.
The ONIX can share map data, the Helix cannot.
The Helix is simpler to use, the ONIX has a more complicated menu.
I prefer the Helix for simplicity and the ONIX for adjustability.
Humminbird Helix 12 SI, 1198c SI, 1198c SI, ONIX 10 SI NT, 360, RC-1