Smashwords does a very nice job with its file conversion; the author ends up getting many, many different formats. However, the site design is a bit clunky, authors basically have to give people direct links in order for anyone to actually find the book, and the work might be surrounded by gigantic piles of erotica (not meaning any offense by this, but there's a lot of that on SW), which may or may not make a difference. The premium distribution is okay; it takes bloody forever.
Amazon DTP has a simple interface with what I believe is the best royalty deal around: 70% (read the terms, it's not without strings). Amazon also has the biggest ebook market at the moment, so the sheer amount of potential traffic is staggering. Case in point: Haven sold maybe 50 copies on Amazon in several months. Last week, Amazon put it to free for 5 days. Haven had 9000 downloads in that time, and, after being put back to the normal 99cent price, has sold 500+ more.
I can't say anything about PubIt at the moment, as I don't have experience.
In the end, I'd say that self-publishing at all places independently is the best bet when possible. Smashwords premium distribution is nice but not anything spectacular, and it takes so long that going to it directly seems quicker and entails less fuss. Since it's possible to opt out of any individual distribution deal, do so with the PubIt and Amazon and upload to those sites individually. However, since there's still Kobo, iBooks, etc, put it on Smashwords as well to get the greatest potential site exposure.