In my experience I'd say both. I joined Smashwords in 2011. After struggling with their formatting I finally got the hang of it and loaded the majority of my books there.
Earlier this year I was on Kindleboards and I heard about Draft2Digtal. I was unsure at the time,I didn't want to move all my books over there without really thinking about it. I did a little research and found out that quite a few people were raving about it. So I decided that with my next release I would try it out. I joined Draft2Digital in May and I haven't been disappointed.
Smashwords has a lot more sale venues than Draft2Digital. It's always important to have your books in as many places as possible. If you want your book to go free at Barnes & Noble you can only do this through Smashwords so before loading a book, think about whether or not you'd like to promo it by making it free. Smashwords Mark Coker is great! He really does have the authors best interests at heart. Every time I sent him an email he always answered. That for me has been a very important thing for me. Yes there are some glitches with Smashwords but with everything it's a learning process. So for now, for books I want to go free go off to Smashwords. Every book tittle is often loaded to Smashwords, I just decide which vendors I'm going to send them to.
What about Draft2Digital? I love Draft2Digital, it's like the new speed train that takes you from one destination to another in half the time. My books are uploaded and appear the very same day at Barnes & Noble and Kobo but apple not so. But this isn't their fault. At the moment Apple is dragging their feet when it comes to reviewing erotica, it can now take up to 25weeks so I'm told but that's another story for a different time. Yes Draft2Digital are the best when it comes to speed. They update sales figures as close to real time as you can get, this is great because you can get to see how each book is performing and make adjustments accordingly. Just today they released a new reporting system which I'm loving. Smashwords is a little slow on the reporting which I find a little annoying.
With Draft2Digital you can create multiple pen names which works to my advantage, unfortunately Smashwords looses marks on this one. With Smashwords you either have to create a publisher account or create a new account under a new pen name. This is a hassle for me seeing that I have a tax treaty with the US. That means I don't pay tax, it took ages for me to get this sorted out with Smashwords. When I finally sent my W-8BEN I had to wait a long time before it was finally sorted. The forms are done in batches. I don't think I have the patience to go through this again. With Draft2Digital this was as easy as pie, I sent my form via email and every thing was sorted quickly, like I said, speed train!
The last thing I want to mention is the payment options. Draft2Digital offer direct deposits which work out better for me. I don't have to endure the paypal fees and the exchange rate is a lot better. I find that with my Kindle sales. Every penny I make is important and to loose some of it to a fees is annoying. But hey things could be worse.
So in conclusion I would say use both. I never pulled any of my books that were at Smashwords and I still load every book to both. I think that having them both is the benefit to authors in different ways. We now have more choices and we can do so much more. It helps me to focus more on my writing and not having to worry about where to load my books.
I hope you found this helpful