Despite not owning an e-reader, I want to buy this. I know that I can download Nook or Kindle apps for PC that will allow me to read e-books on my computer. Is there any difference in the formatting/layout, or any other reason, to purchase the Nook version over the Kindle (or vice versa)?
If all you care about is the content, both programs will work fine for most novels. (If there's any maps or other images, though, the Nook version will have higher-quality versions.)
But while I don't know if this applies to "The Struggle Within" in particular, the Nook version will allow for prettier formatting, custom fonts*, etc. that the Kindle version can't have.
* I'd bet that it does
have custom fonts, simply because that's something S&S has become fond of over the last few months.