If you want to take notes or highlights on the same device you’re using to read, then definitely use the iPad. If you just want to read, either the iPad or the Kindle paperwhite are fine.
I have an ancient iPad mini, and an antique Kindle paperwhite. The iPad mini lives by the couch in the living room, and Kindle in the bedroom.
Like @acavender post above, I have both (well, Kobo Libra now, not Kindle) and like others, I think I’ve come to the conclusion that again that the Kobo is better for fiction and the iPad Mini for non-fiction.
Whilst you can make notes on the Kindle/Kobo, I’ve found the refresh rate of the monitor to be a bit of a pain - it’s not difficult but it’s just not as easy as the iPad. I think either will suffice for you really whichever you decide.
The PaperWhite is an amazing reading device: light, reflection free, no distractions.
That said, if you’re reading to take notes, typing on the kindle is so frustrating.
So, if you’re reading fiction for fun, get the Kindle. If you’re also going to be taking notes (ie, more than highlighting), then I prefer the iPP with keyboard.’’
I also have the first gen Remarkable which I never use. Should probably sell it. It’s a great idea, but I haven’t found a compelling reason to use it.
I have a Boox eInk device. It’s a joy to read off, especially in the sun but the backlighting is good too. I much prefer extended reading on eInk.
An iPad might be a better device if you note take extensively, although I highlight a lot on the Kindle app without problem (or at least no more problem than my iPad).
I did have a Kindle, but the Boox is not limited to Amazon. I have the Kindle, Libby, Google Books and Perlego (excellent academic book subscription service) apps installed on it. It also has a built in eReader, although I tend to upload to a cloud service so I can sync reading with my phone.
I also have my Bible/Logos apps installed.
Apple Pencil FTW. But just as another potential option, if you’re going to find yourself reading in low light I’ve found the lighting on the Kindle is less irritating to the eyes than an iPad - and the lighting on the Kindle Oasis is much better IMHO than the lighting on the Paperwhite. But obviously the Oasis is more expensive. 
I use an iPad Mini for most of my ebook reading. My spouse has a Kindle Oasis that I occasionally use.
I, personally, prefer the Mini. The screen seems sharper and more vivid to me (maybe it’s just what I’m used to), I get color where it’s available (she never knows what the covers or some illustrations look like), and interacting with the book for highlighting and moving around is easier. I always find using her Oasis a mildly painful process.
The Mini also lets me easily use books that I haven’t purchased from Amazon. I have quite a pile of ePub books that were freebies or purchased elsewhere. Yeah, there’s Calibre for conversion and I guess there are changes coming to Kindle there, but right now it’s just more overhead that gets in the way of reading.
One drawback to using the Mini for Kindle books is that the app isn’t great. I like the reading features but sometimes it freezes on startup, it’s painfully slow on older hardware, and it regularly takes me to an Amazon “books we’d like to sell you” screen instead of to the book that I was reading. Grr…
Oh - watch out for the Oasis. It’s got a small battery that, on hers, died early (outside of warranty but not by much). In more recent models the battery isn’t replaceable. Would not buy again.
I have the Kindle Paperwhite (v10, fairly recent) and an iPad. I use both for reading. They both have advantages, just as an iPad has advantages over a laptop - and vice versa.
If I am reading purely for pleasure where notetaking is not a priority, I prefer the Kindle Paperwhite to the iPad. If I am reading where I want to make meaningful/more substantial notes — more than just highlighting — I prefer the iPad. For example, I’m reading Tiny Habits right now on the iPad specifically because I’m highlighting and adding notes to my highlights. [What a time in which we live where we can dictate our notes using Siri!] I’m also using Drafts as I’m building notes by Copying/Pasting from the actual text. I’ll add that note back to the Kindle notes when I’m done with the chapter.
I prefer the Kindle for leisure reading. I have issues, however, when accessing my Libby books (from the library) while staying in some hotels. If they require you to log in on a web browser, I frequently run into roadblocks with the “experimental” (for many years) web browser.
He said he already has most of his library in Kindle, so probably not good to switch to iBooks.
When I bought the iPad Mini, I thought, my kindle oasis will gather dust…. I was mistaken.
Yes the mini is the lightest iPad BUT it still is heavier than the kindle.
More importantly, the iPad display is no match for the kindle oasis display. So much easier on the eyes and not as bright which is perfect for night reading.
Finally, I find that reading on the iPad keeps me up at night while the kindle doesn’t. I think the bright display tells my brain it’s not sleeping time yet.
This is why the KINDLE is my reading device of choice for books.
(Note: I read for enjoyment and mainly fiction)
my take is that Kindle ereader are great for books with mainly text but iPad mini is better for books with graphics or text with images that needs color differentiation
Also some prefer the Kindle for ease of reading in sunlight and also late at night (to avoid the dreaded blue light that is said to keep one awake).
True.
But you can read Kindle books on both iPad and iPhone (which I do frequently) and a message pops up on the second device asking if I want to go to the most recently read page on the other device (when it is further along).
Oh, and I’ve just remembered a detail that you might find important.
If you read a lot of research and scientific papers with double-column layout, you might find the iPad mini too small for reading comfortably, without zooming in or straining your eyes. When the latest iPad Mini came out, I was really tempted to buy one for the exact purpose you’re considering, but I’ve chosen to stick with 10”/11” iPad. It might be a good compromise between the nimbleness of the Mini and big canvas comfort of the 12,9”.
Yes my point exactly. I was responding to someone who said he could start using iBooks. Kindle on iPad and iPhone is a great companion to my physical Kindle, and I use them often.
This is – if you can afford it! – the correct answer for most people in this forum. For most gear questions, in fact. 
This is a great write up of the pros/cons.
That was kind and thoughtful, thank you!
+1 on the Boox.
I use a Boox Leaf which enables me to use the Kindle app and read my Matter read-later queue in the Matter web app.
Are you going to be reading outdoors? For that use case, I would go for a Kindle or, better yet, one of these bigger ebooks (Remarkable, Onyx, Boox…) as long as it can run the Android Kindle app. I have an Oasis and my tired eyes would prefer somewhat slightly bigger. The Kindle app supports cross-device highlighting and I’m sure there are solutions to download the highlights to DEVONthink or something like that. While highlighting support is adequate, taking notes on a Kindle is definitely only for urgent ideas.
If you happen to have more important things to do while outdoors, an iPad Mini would have the bonus that you do not lose your Apple-based workflows. But that can also be a distracting factor…