Smashwords was taking forever to get “technical integration” from Amazon, and when I got a query from a friend about Amazon availability, I decided to go with KDP (Kindle Direct Publishing). Amazon’s registration process isn’t more painful than you’d expect, given that they need to pay me and report my income, and the submission process gives me more control than Smashwords’ does, though it takes more work to take full advantage of it. (The best way to submit a book to KDP is as an HTML file with detailed CSS, and saving as HTML from OpenOffice gives you that. I had to make some manual changes to the CSS for a good result.) This means there are some differences in formatting between the Smashwords and KDP editions. There shouldn’t be any differences in content.
I’m not thrilled with Amazon’s commitment to DRM, closed platforms, and licensing rather than really selling e-books, but I don’t dislike them enough to cut myself off from that market. So if you’ve been holding out for the Kindle version, wait no more!