The 6 Plus is just about exactly half the size of a Kindle Paperwhite.
Which raises the possibility of a Kindle Paperwhite. Under a hundred bones. You should get one. Best way to read a book short of reading a book IMO.
I have an iPad, a 6 Plus, and a Paperwhite. Between the iPad and the Paperwhite, I prefer the Paperwhite. I don't need to go into why eink is better for sustained reading. But between the Paperwhite and the 6 Plus, I use the 6 Plus way more. Why? Because it's always with me. And I know it's already charged. I know I can sync the books anytime, and download new ones without worrying about being connected to wifi (or dealing with the super slow connections trying to navigate a store or menus on an eink screen). The text looks fantastic on the 6 Plus screen, and I don't have to scrunch it up to my face to make it readable. The 6 Plus has me reading more again, because I can pull it out anytime I have a free moment and read a few paragraphs or a chapter out of a book. It's great for that.
If you're reading ebooks at home, I highly recommend a dedicated eink e-reader, but if you read them mainly on the go, the 6 Plus is the perfect device for it.
As for an iPad and phone vs just the phone, it depends on your usage. I have an iPad Air and I'm not giving it up even though I have the Plus because the size difference is significant and it's perfect for reading comic books. I read a ton of comics, and the Air is close to the original print size, so I can read them with little compromise. I can't do that on any other device that runs iOS currently.