I have 5 tablets (and a smartphone) and a Pocketbook 302 ebook reader. The only time I use a tablet/phone to read on is when I am waiting in the doctor's office or similar. I use the ebook 2 hours or so a day, every day, and I can spend several days (when camping) doing nothing but reading (so 15+ hours a day).
The BIG problem with tablets is the battery life. I can go weeks between charges on the ebook. A tablet will last me 3 or 4 hours tops. I can use an external powerpack (and I have 4 of them), but that only adds another 3 hours or so. Basically - a tablet will be pushing it to last you one day.
The other big problem is reading tablets/phones outside. You basically can't.
The battery thing alone kills it for me. I simply cannot be bothered organising myself to have external battery packs, etc, so that I can simply last out one whole day. I have 5 tablets just because of this - I have 2 or 3 on charge all the time, so that at least one is fully charged at any one time. I now mainly use the tablets for Reddit, FB, ebay, and things like that. Not for reading novels. This is not a problem with the ebook. Every week or so I will check the battery level - in 4 years I have only ever run it flat once. The ebook will easily last me 2 or 3 weeks between charges.
Also - tablets have an OS (so do ebook readers, of course, but much less stable) . They crash, need rebooting, etc. My ebook reader ALWAYS works. I just turn it on and read. Android is as buggy as all hell. Dunno about the Ipad.
Just my 2 cents worth