You're not "locked" into amazon only for getting books. You can get books from a number of other online vendors. The penguin issue will not go on forever, there are so many other books to read! Also, you can read Kindle books on your Blackberry, PC, Mac, Ipod touch, Ipad and other devices.
I found while testing out my Kindle with the Nook @ B&N that the Kindle was a lot easier to work with; with the nook it takes far too many steps to make a simple highlight or note. I use the notes feature a lot and I love having an actual, physical keyboard as opposed to a virtual one- some people find the virtual keyboard decent enough for a note, but I had far too much trouble hitting the right keys.
About the number of books available, you would be correct in that most of B&N's "million" ebooks are out of the google public domain catalogue, pre-1923 books. I heard it somewhere said that up to 750,000 of B&N's million ebooks are from Google, which means B&N has only about 200,000 something recent ebooks available. On the other side, Amazon have over 500,000 recent, in copyright works available. That's a sizeable difference.
All those google ebooks are available (with far less ocr errors and typos) via websites like the Gutenburg project. And if you do need those google books, since they are drm-free you can convert them easily to be read on the Kindle.
But at the end, it's up to you. You do have a month to return the kindle for a full refund (I think you only get 14 days with the nook, with or without a restocking fee). Take your Kindle to B&N and try them out side by side. See which you prefer.
(And if you think the nook's screen contrast is better than the kindle's remember that the nook has a black border around the screen, that the kindle lacks which optical illusions your eyes into thinking one is better than the other. A dark colored skin will even it up between the two devices- the screens are made by the same company after all.)