Going direct tends to give you more control. For example, if anything goes wrong when Smashwords sends your book to Amazon (eg. the cover is missing) you'd have to ask Smashwords to fix it, then wait for them to get around to it, when you could have fixed it yourself. Plus you'd have to wait for Amazon to send sales numbers to Smashwords, whereas if you publish directly through Amazon, they'll report sales numbers in real time via the KDP Dashboard. In addition, Smashwords will take a cut of your Amazon royalties.