I think the first thing you should do is realize that you're choosing between excellent devices so there is no wrong choice. Tossing a coin will work for you.
I haven't used a Clara HD although I've used several Kobos. I am considering getting a Clara HD but I haven't decided yet.
I have all the Paperwhites except the 2nd one, and I had one of those but gave it to a friend. I have 2 of the Paperwhite 4's. Please don't get the silly idea that I'm a gadget freak.
I do have a fairly strong preference for Kindles for a few reasons. First is that the firmware on the Kobos is a bit buggy. No show stoppers but things don't always work the way you hope they will. I currently only have older Kobos and I read with them more than a little bit but they aren't problem free.
Second is that I find the hardware a bit less reliable. About half the Kobo's I've used with slide switches were hard to turn on and off. One that I have now won't turn on or off so I use the sleep cover, which I'd do anyway so it isn't a problem. Over the years I've had a lot of Kobos and Kindles. I've had one Kindle battery go dead after about 3 years but no other problems. I've had small problems with a lot of the Kobos. Of course the newer Kobos no longer use slide switches.
That said, if the only reader I had was a Kobo I'd be okay with that. It wouldn't be what I'd prefer but it would be okay. There's much that's good about them too.
Kindle firmware does what you expect. The worse thing I can say about it is they overdo some of the features making me have to stop and think about them where on the Kobo I just use them thoughtlessly.
I did have one customer service issue with Kobo and it was nearly impossible to get them on the phone. It took a few weeks. I'm retired so I could try during business hours a few times a week but it was very, very difficult. Once I got them on the phone they handled my situation nicely and were eager to help.
With Kindle support they're on the phone with me in less than a minute, always. They're helpful and inventive and a pleasure to deal with.
I've never bought Kobo books or used Overdrive so I can't help with that part of it. I buy my books from Amazon and covert them to use on my Kobo. That's never given me any kind of problem. It just works. However, not all the features work on side-loaded books. Most do, most of the time. That's always been okay. The features I need always seem fine.
I'm sorry if I'm making this more difficult for you, but the only real reason to pick one over the other is personal preference. I think the Kindle has advantages but nothing very important.
By the way, even if I didn't have a Kobo I'd convert my books to epub. i want backups and epub is a good flexible backup format. I don't expect Amazon to go under tomorrow but decades ago I bought B&N's ebooks (years before the Nooks) and one day they announced they would no longer be in that business and that any books that weren't downloaded within 30 days would be lost. I downloaded my books, maybe 15 or 20 of them. A month later I bought a new computer and their reading program was no longer available so my books were lost. I'd already read them so it wasn't a tragedy but it taught me the value of backing up books.
Anyway my point is that not having to convert my Kindle books wouldn't give the Kindle an advantage for me since I'd convert them anyway.
Barry