I have Kindles, Kobos and Nooks. I don't know much about the Onyx or Pocketbook readers. My personal preference is for the Kindles but I also like the Kobos and Nooks.
One advantage the Kobo will have for you, and I'm not sure this is true of all the models, is the ability to change the light color. I have this feature on my Nook but I don't use it. I've tried it and I prefer the normal color. A lot of people disagree with me on that.
Anyway that also lets out most of the older ereaders that you might find used. This is a fairly new feature.
Both the Kobo and the Kindles come with very good dictionaries. The Kindle dictionary seems a bit better but both are very good. I use the dictionary a lot and I'm content with either of them.
The Kobo tends to be a little buggy. I don't buy Kobo books. I convert my Kindle books and read them on my Kobo and that might be why. Most of the features work just fine for me most of the time. Sometimes some feature won't and I rarely know why. I don't see this as a serious problem.
On the Kindle everything pretty much always works. Both have more features than I use. Both have all the features I care about although I do like the X-Ray feature of the Kindle, as well as being able to sync with my phone's Kindle app. But these are minor things to me I can sync manually almost as quickly and I mostly use X-Ray as a character look up for characters I've forgotten. I can do the same thing on the Kobo using text search.
There is a difference in battery life. When I'm reading they seem about the same. When I set the Kobo aside and don't use it for a while it holds it's charge better than my Kindle when I'm not using it.
My Kobo is an older model so comparing screens doesn't make sense. I have used a more recent Kobo when I set it up for a friend and showed him how to use it. It's screen was just fine. As good as my Kindle's screen.
As I said I don't know about the other brands but choosing between a Kindle and a Kobo for most people is simply a matter of personal taste. Both are good. I'd say pick the one you feel best about.
Barry