Using Calibre to read a book you're legally entitled to read on a different device may be considered immoral by some, just as complimenting someone when you're not sure they deserve it is lying and is immoral. Some sins are really worth doing.
The key is to not get books you aren't entitled to. Which device you use shouldn't matter.
As for Kindles and Kobos, I have both. My preference is for Kindles both both are excellent devices and worth having. My Kobos are older models and get used a lot and have had no hardware problems at all.
I live in a retirement home and I've also helped a lot of my neighbors get ereaders, both Kobos and Kindles, and none of them have had any problems.
Actually the only problem I've had with one was recently when an older Kindle's battery became kind of flaky. I just replaced it with a slightly less flaky battery from Ebay.
My suggestion is to read up on the features of the two brands and get the one you prefer. They're both excellent.
Barry