I had a nook (which I sold to buy my iPad) so I will say that eInk is something to be reckoned with. It's very easy on the eyes. However, iBooks is an AWESOME app. Turning sepia on reduces eye strain, I love the highlight feature, I love the tap to define feature, and I adjusted to reading books on there vs. the nook quite nicely.
I say the iPad, in as unbiased of a way as possible! I assure you you won't regret it. It'll exceed your expectations.