I use an iPad mini for reading and web browsing. I read and web browse on an iPhone, but web browsing is more enjoyable on the iPad. I also use the iPad mini to test an app I'm developing which can simulate an iPhone screen as well as behave like a regular iPad. I use Pages for basic document creation. But I do find that working on the iPad decreases productivity so anything substantial I'm working on, I often need to return to an iMac.
I use the iPad mini usually on the couch or on a table, at the library or on the go. One reason I chose the mini was due to it's very neat compact size and much lighter weight than the larger iPads. Plus, the mini doesn't have a protruding camera. (I think the camera sticking out on the new iPad Pro is such a poor design decision, but that's just an opinion.)
I suspect the Kindle with e-ink has much better display quality for reading in super bright light such as under the sun at the beach. The iPad won't be as great in super bright beach light. But I guess this all depends on if your iPad likes to stay seaside under the sun often.
I was attracted to the mini because although the colour gamut technically wasn't apparently as good as an iPad (which I haven't really been able to notice), the image quality was incredibly sharp, even more sharper than iPad when comparing them side by side. (This would likely be due to the higher pixels per inch (ppi) with 326 ppi on the iPad mini just like on iPhone vs a lower 264 ppi on iPad Air and Pro.)
For ebooks, and other resources in the typical ebook format, mini is great, and I can adjust the text size when reading in the iBooks app. Sometimes when reading A4 sized PDF's though, due to the text size being fixed in a PDF, text can be rather small to read on the mini. However that's easily fixed by a double tap on the block of text to zoom in and focus in on that piece of text, or rotating the device landscape to fill the width also works a treat. Sometimes when reading PDFs, and depending on the set up of the PDF document, I get slightly strange colours due to the way the document was created either in an RGB or CMYK colour space (e.g. dark blue displays as lighter cyan blue). It's not an issue for most things, just some PDFs.
One small gripe is when web browsing with Safari, it can crash from time to time, and sometimes it needs to reload tabs. (It's something Apple needs to look at improving for future devices to completely eliminate the issue.)
When approaching the purchasing decision, I usually go with the lowest capacity - 16GB. (Side note: actually Apple should build in the highest capacity these days considering storage costs Apple very little.) I simply back up the device and at the same time free up storage during that process, but storage rarely gets out of hand like that. I chose the Wifi model which is fine for almost all places. And, in the event there is no wifi, I turn on the iPhone's Wifi Hot Spot in settings and use the internet through the wireless iPhone connection. Simple and useful. I purchased the iPad mini when a special discount sale was on at a local retailer.
A lot longer than I originally planned, hope it helps!