If you live in US, want connectivity from your e-ink reader and don't mind the physical keyboard -- I don't think there's a better device than Kindle.
If you hate a physical keyboard, travel internationally and don't want to pay to be able to download your stuff while overseas (or have a weak mobile signal in your house) or have faith in Android and community -- get the nook.
If you already have a Sony PRS reader (especially 505, 300 or 600) I don't see why would you want to change a reader now tho. Considering you've already managed to acquire and load content for your reader for quite some time, why get another gadget?
I did tho, I did get a nook, because I have a faith (well, I'm also a nerd, so why not?).
