I quite frequently use the iPad to read books in epub or pdf format, and find it very comfortable to use.
The big advantage of iPad is all the extra's it provides, it's not just an e-reader, but an actual computer with lots of different features. It excels at surfing. But for example, I also love to use it as a media center client. It can be used to take notes, chat with friends and family, etc etc. This advantage is also disadvantageous in your case - the iPad isn't built primarily as an e-Reader; it's built for a lot more. So there are some disadvantages you may encounter when going for an iPad.
For one, an iPad is
heavier than an e-Reader. I personally have had no problems with it, an iPad is quite easy to hold (though most of the time I use a cover to support it for easier reading), but I never experienced a e-Reader.
Also, the glass makes it nearly impossible to read your books in
sunlight because of
reflection. I personally have had none issues with this because I always read inside, however it may be an important thing to consider. Some e-Readers use eInk technology which is far superior to the LCD technology in the iPad regarding the reflection problem and also
"eye strain" if reading for a long time. Again, I have no problem with this, but most people are actually susceptible to eye strain with LCD screens, so be sure the try and read for a long time on a pc screen, and see if your eyes get tired. If they do, you may like the Nook's IPS screen more.
In the end, you should look at what you need. As solely an e-Reader, I think that the Nook (or other e-Readers) will excel most. The iPad however offers much more of a complete experience (the web, surfing, mailing, chatting, ...), and does indeed, as pointed out by tmarks11, offer access to nearly all book publishers (contrary to the Nook). That's a point iPad excels in again.
Good luck with your choice!
