Reading indoors isn't so bad on backlit tablets
My wife has migrated to an iPad, so I can tell you that it's definitely doable. We moved her whole library over from an older Nook via dropbox. And her eyes are a lot more sensitive than mine for glare, etc. For basic black/white text reading, the Nook/Kindle are just as good however. Anything else and you can benefit from having the horsepower/better screen of an iPad, etc. As Bob said though, bright light reading is a no-go unfortunately.
Unless your collection is multiple gigabytes (which for most folks is unlikely), your whole library should fit perfectly.
The Android tablet & iPad mini will both handle all types of ebook files from different sources, although it will require an app or two to get it just right. What constitutes 'most comfortable to hold' is still very subjective. My wife's full sized iPad works just fine for her needs- I don't see why a smaller tablet couldn't work as well. YMMV.
FWIW, you would not be limited to content from Amazon if you went with the Paperwhite, btw. It would merely mean you would have to convert certain types of ebook files found elsewhere in order to view on that particular device.