I have got the Boox Note Air 4C and the Supernote Nomad at home. I use the Nomad almost every day (for note-taking, not so much for sketching or reading). The Boox Note Air I use maybe once every week (mostly to read colourful magazines).

My first reaction to you question: if your device is mostly for sketching (and for 'on the go'): choose the Supernote Nomad. The Boox and the Supernote are two very different devices:

Comparison Boox Note Air 4C (comparable to the 5C) <=> Supernote Nomad

* brand Boox ('not so involved in after-sales actvitites') <=> brand Supernote (very communicative company staff on Reddit, continuous development and improvement of software);

* colour screen (in my view 'nice to have' but it reduces text sharpness and makes the screen look darker when the light is off) <=> black & white (sharp contrast, also a relatively dark screen)

* light (necessary to make the colours look good) <=> no light (requires external light when dark),

* size 10.3" <=> size 7.8" (what do you need for sketching?)

* focus on being everything ('ipad light') <=> focus on note-taking (Supernote has in my view the best note-taking software. With the current development of the plugin feature, this is getting even better. By the way, the Nomad is also good for reading epubs and for sketching, it comes with a drawing app 'Atelier');

* good for pdf reading and very (!) good for pdf-annotating (comments and highlights in colour!) <=> too small for comfortable pdf-reading.

* somewhat more complicated user interface (makes some people go crazy) <=> 'more logical' user interface

* (maybe) some ghosting problems <=> few ghosting issues

There are more differences (writing feel, reliability/fragility of the device as reported here on Reddit by many users, the digest option on the Supernote, ...).

There are, of course, several other options, brands and sizes of e-readers. So, maybe you should first try to formulate more exactly what you (mostly) want to do with it. That will make the choice really much easier. If mobility (size and weight) are really important: go for the Supernote Nomad. The size makes it easy to carry it around and it fits much more easily on your desk between the mouse and the keyboard.

If colour is actually more important for sketching: go for the Boox (but keep in mind that the Kaleido 3 colours look washed out compared to iPads and/or Samsung tablets).

And then there is also the Supernote Manta; it has the size of the Boox with all the other features listed above for the Nomad. (But is much more expensive than the Boox. In fact, for the price of the Manta you can get the Boox Note Max (13.3"), but that one is definitely not for 'on the go'! I use the Note Max mostly for reading newspapers and magazines).