You'll probably find these two side-by-side at your local bookstore, as the popular wisdom seems to be that these three are in the same practice of writing books on what seems to me to be best described as "pop-sociology," although Levitt is an economist and Gladwell is a historian. The reality, however, is that in this particular field, Gladwell, as hard-working, prolific and intelligent as he might be, is not fit to shine Levitt and Dubners' shoes.
The difference between Levitt/Dubner and Gladwell is the difference between Rocky Road and vanilla ice cream. Rocky Road isn't for everyone, but it certainly has a lot of flavors to consider, while vanilla is safe and digestible, but ultimately, somewhat unsatisfying. The eater feels like something is missing.
So it is with Gladwell, the Bing Crosby to Levitt & Dubners' Mick and Keith. Gladwell is much less likely to stir controversy, his conclusions in books like Blink, The Tipping Point and Outliers are nearly universally hailed as groundbreaking and revolutionary, and very rarely challenged in popular media (perhaps despite Gladwell's protestations to the contrary). On the other hand, Levitt & Dubner, with assertions such as that there is a causal link between abortions and drops in crime rates, are regularly excoriated by both the right and the left.
My own feeling is that what is missing from Gladwell's work is some kind of practical application; indeed, Gladwell doesn't even seem to be looking for one. While Levitt/Dubner's work easily lends itself to all sorts of ideas about how we can change the world and why we do or don't, in every area from education to climate change, Gladwell's work seems more about wry observations of why things are how they happen to be, with little concern about what that means for the future.
Freaknomics (and SuperFreakonomics) tells us why and when people cheat and how people tend to over and underestimate potential dangers in the world. This is information that is very clearly useful, and can very obviously inform our future behaviors, if you buy into it. Gladwell's work tells us that in certain cases, people born in certain months have a greater chance of success (Outliers) that people sometimes instinctively know when something is out of place (Blink) and that certain products blow up in sales when enough of the right kind of people take an interest in it (Tipping Point). It's all interesting to be sure. But is it really groundbreaking and revolutionary? Without some kind of context, some interesting ideas about how we can manipulate such information in a useful way? Not for my money.
Speaking of my money, I bought both SuperFreakonomics and What the Dog Saw. Sure enough, Levitt and Dubner jump right in with the both controversial and useful notion that, in deaths per mile, driving home drunk is many, many times safer than walking home drunk, and moves into the idea that television empowers Indian women while at the same time leading to an overall increase in crime (not violence, but crime) all over the world, while Gladwell starts us off with story of Ron Popeil, what makes him special (he learned to make the product the star) and how he came up with the idea to invent the RonCo Rotisserie (he realized that people pay a lot for rotisserie-cooked food) and moves on to explore why Heinz has cornered the market on ketchup (they make the best ketchup).
To be fair, the Gladwell book is a compilation of some of his favorite New Yorker articles, rather than a single book concept with a unifying theme, and as he states, is not intended to persuade, but simply to give insight into how others think. It is also well-written, and well-researched, and Gladwell is to be commended for this. The Ron Popeil story, the quest to make a better ketchup, and the other stories Gladwell tells are interesting, in their own way.
That being said, if you want to read a book that will really keep you up at night thinking about things like the economics of prostitution, how terrorists are profiled and whether or not you should be more afraid of elephants than sharks, you're going to want to plunk your money down on SuperFreakonomics. I'm sure Rick James would agree.