If you're looking for a one-size-fits-all model for distributing your book, then you need look no further than Draft2Digital and Smashwords. Well, maybe a little further, because in both cases you should still use KDP for distributing to Amazon. It simply isn't worth using a third party system for loading on Amazon, because KDP is very easy to use and is quite a bit more powerful than using a third party system to sell your book on the world's largest book marketplace.
But, trying to maintain multiple platforms (like Apple, Kobo, Nook, etc.) individually and maintain a different file for each upload is a nightmare, which is what makes these systems so nice.
All three companies offer excellent distribution options, with Smashwords being just a little bit ahead in the number of options it lets you distribute to. D2D is constantly adding new markets, and both of those networks have stores available that you couldn't do on your own as an indie author, but Smashwords still has just a few extra markets that D2D isn't in yet.
One clever thing that D2D does is allow a user to have a paperback created and distributed as well as e-books, which truly makes this a one-stop shop for releasing a book. You could hypothetically use the version loaded on CreateSpace to make an e-book, thereby releasing in almost all markets.
Also, even with this service, I would never actually use it to make a paperback copy because I prefer to customize the documents I release way more. So, it's nifty, but not actually useful.
Ease of Use
Smashwords feels clunky and old, making it difficult to use. D2D, on the other hand, feels sleek and modern and looks really nice. I've use both a modest amount, and I have to say it is a pleasure to work with D2D and a nightmare to try and figure things out on Smashwords. D2D just feels clean and looks pretty.
It is just easier to use D2D to create a book, and their website is so much better. On top of that, they've build an entirely new ePub and Mobi building system that lets you build really pretty files that you can use anywhere, and it is second to none with how easy it is to use.
Smashwords offers a super neat system for creating coupons and giving your book away where people can grab it straight from Smashwords. This makes giving away review copies cheap and easy. They also have their own marketplace where you can sell your book directly.
However, it feels very much like a third party website and poorly integrated store, and I have seen many reviewers who are not at all interested in receiving Smashwords Coupons, and it can come across poorly.
Draft2Digital offers the ability to add meta information (dedication, cover, rights page, ToC) directly, as well as a simple upload feature where you give them a word file and they will create all of the necessary formats for you.
Even more, they offer a newsletter for releases that people can sign up for inside your books. You simply give them permission and they will collect email addresses for you, and when you release a new book they can send an email notification to all interested readers for you.
Draft2Digital just started rolling out new features for this, including the ability to extend this newsletter from the website Books2Read and even give launch notifications across platforms they don't support. This is really cool and works somewhat like Amazon's follow system, though much less powerful.
In either case, Draft2Digital is the clear winner here in extra features that are sure to become smoother and better over time.
Overall Impressions of their Differences
You can't really go wrong with any of the aforementioned systems for handling wide distribution, but if you want my recommendation, turn to Draft2Digital first. As for D2D and Smashwords: both take the same cut, with D2D will save you a lot of time and is just a pleasure to use. Also, Books2Read is a growing sister website that could one day serve to sell more books, and having a head start with it could be hugely beneficial to any indie author.
Smashwords distributes to more places than the other, but not by a lot, and Draft2Digital has been working to steadily close that gap in recent months.
Your best bet, honestly, might be to use all three at the same time. It isn't easy to do, but there are some tricks I've learned that will make it a lot easier than trying to go it alone!