From my research, I don't think the screen differences are a deal breaker or maker. What I found (for me) is that I much prefer the real keyboard on the Kindle to the virtual KB on the Nook.
Kindle does not do epub without some file manipulation. Nook does epub natively. Most library books seem to be in epub format.
There are tons of books for both and you are not restricted to just the Kindle store or to B&N's store.
K3 is lighter in weight and has longer battery life than nook. Nook has a color option with the Color Nook.
Really, you should go to the store and put your hands on them and decide what works for you. Some love the touch screen controls of the Nook. Others, like me, prefer real buttons.
I just almost pulled the trigger anyway on a Nook when they were offering them for $99 recently. But I wasn't ready to buy at that time and they sold out... which made my decision to go with the K3 easier.
