Nook is open source, which means you can put and read documents other than proprietary ones on it (e.g., any pdf's for work/school, also library e-books which are free instead of books that you buy from Amazon). This makes it way better than the Kindle. If you already have an iPhone and a netbook, you don't need an iPad it would just be redundant. Its mostly about branding anyways. I'd get a touchscreen computer before buying an iPad.
Well, there are several opinions masquerading as fact here, so I'll add my two cents.

First, you can send PDFs to your Kindle and they can be read on your Kindle. Neither the Nook nor the smaller Kindle is really very good at PDFs - the larger Kindle DFX is the workhorse there. It is true you can't download library books onto a Kindle - most libraries aren't using the format that Kindle supports. If library books are important to you, either get a Nook or download an app on your iPad and read them that way. I can't remember the name of it now, because I don't want to check out ebooks form the library, but there is an app you can download that allows you to check out books from the library.
Neither of these functions make the Nook "way better" than the Kindle.
Also, proprietary is in the eye of the beholder - B&N books are just as proprietary to the Nook as Amazon ones are to the Kindle. You can't read B&N books on a Kindle, either.
I have an iPhone and a netbook and I don't find my iPad to be redundant at all. In fact, it's the netbook that is redundant now. The iPad does everything I wanted the netbook to do, and it's smaller, more portable, and offers more features. It's a fantastic magazine reader, too!