The best way to understand really is to install both and try yourself, but basically I would say Kakoune is more "radical" than Helix, which feels more like Vim. Both move the selection in normal mode, but Helix has you extend it using what's basically visual mode, whereas Kakoune cuts out visual mode altogether and has you hold Shift. As you can see in the config, reconfiguring what Shift does causes issues with normal Vim bindings (like joining selections with J), so Kakoune solves this with Alt.
After using it for a few days, it made a lot of sense to my brain—I would say, in general, Kakoune feels enormously well thought-out and carefully considered in every element of its design.
I think it's also worth pointing out the social factor in pen/paper notes as well—jotting things down on a notepad seems a lot more attentive than typing into your phone.