A little less than a month ago, the Kindle for Android app came on the scene, much to the delight of the Google device-inclined. Although Amazon's app is clean, simple and rather stunning (much more attractive than its iPhone counterpart and Borders's Kobo-powered app, IMHO), Barnes & Nobles's app has a few more goodies.
First of all, the Nook boasts eight font styles and five text sizes -- which means I can read Imperial Bedrooms in Times New Roman (no Comic Sans, alas). The Kindle app, meanwhile, only allows you to adjust size, brightness and background color -- white, sepia and black. In addition, you can turn pages in the Nook app by flicking your finger (or turn on page animations), which is a much easier reading experience than the swipe function that the Kindle employs, in my opinion. The Nook's navigation bar is much simpler than the Kindle's as well, allowing one to easily scroll through the book via page numbers.
Like the Kindle app, you can save and sync bookmarks across devices, but the Nook app also lets you share e-books with friends via LendMe, which allows folks to lend books to any device that will run the B&N eReader software for up to 14 days (you yourself can't read said book during that time, though, which is a bummer -- and it's a one-time share). With more than 1 million titles available in the store (which you can shop for in the app), there's a lot of reading/lending to be done -- the Kindle app, on the other hand, has 620,000 titles.