For me it's always going to be function over form. What makes me pick a Kobo over a Kindle is what's inside... how my reading experience will be improved. Such as until recently, Kindles didn't have a way to darken or thicken fonts, which made for a terrible reading experience for my eyesight. I have no idea why Kindle was so slow to add that, even my Sony 350s were capable of it from over 8 years ago, and a feature I can't do without. I have a Kindle PW2, but I use it to just download a free book each month, I never read on it.
The majority of people use covers on their readers to protect them, so anything fancy done to the back just vanishes, making it a rather pointless piece of eye candy. Even the front, you'll only see it while reading, and a busy or fancy bezel I find just distracting. You could just add a skin to the front of your reader if you want to customize it for yourself.
As long as it's not a ridiculous pink (sorry to all the pink lovers, but it's not for me
) or even white, I prefer just a standard black frame. The darker frames actually tend to make the text appear with a bit more contrast. Shiny bezels are out too (Nooks were horrible with their white shiny frames, no idea if they're still the same, I haven't even looked at them for years, but it was an instant no due to that).
I did love the Sonys with the very dark blue frames that were only available in Europe for some idiotic reason, but I did manage to get 2 of them used that are still perfect. If only they had a front light, I'd still be reading on them.