I buy almost all my books from Amazon. They get hundreds of dollars a year from me.
Sometimes, however, there is an exception. For example, last month (?) Kobo had this very unusual 90% off. I had never seen this before. I took advantage of that and bought about 30-odd books.
If you make a mistake on a purchase with Kobo, however, it'll be like being caught in a Kafkaesque world of brightly-lit mirrors and revolving doors, of shadowed ebook bureaucrats with gleaming teeth and razor-sharp mustachios. The language they speak will, unfortunately, be one that does not exist anywhere on earth, except perhaps in the Halls of Kobo. You will never see them face-to-face either, but through the constantly shifting mirror-images that flicker to life and disappear. These constantly shifting forms will coalesce and then drift apart, an endless series of flickering images that can never be finalized as a person you can actually talk to and see. At least, that's been the experience of a few people here who've attempted to crawl through the barbed wire....
Amazon, on the other hand, will be as painless as removing an eyelash dangling from the tip of your nose. At least, that's been my experience.
Remember: Experiences DO vary, and yours may not correspond to the ones I have described.
Calibre is my good buddy. He's as close to me as a pair of newly-washed underwear.