I have a Nook and I'm just waiting for holiday price wars to buy a K3. I'll probably keep the Nook for the occasional library checkout but, as good as my local library is, the eBook collection is mostly romance and self-help
Biggest reason I'm switching. It turns out that my non-fiction and boutique publisher reading habits are not supported by B&N. I continually run across books only available through Amazon. The second biggest reason -- I have a huge preference for the speed and great search functions of the Amazon website. I find B&N's website to be very clunky and slow with poor search ability. Your mileage may vary
A good way to figure out which you prefer. Download both Amazon's and B&N's free eReader software to a device you already own like your home computer or smart phone. Then start filling them with free sample chapters and free books. Then when you purchase an eReader, you'll already have content for it and you'll know that you picked the web storefront that you like the best.