However when the dataset is medium sized (i.e.: fits into your computer's memory / 2) R crushes Python (and Pandas) for the 80% of the time you'll be spending wrangling. The reason is that R is vector-based from the ground up. Pandas does everything that R does, but does it in a less-consistent, grafted-on way, whereas the experienced R person who "thinks vectors" is way ahead of the Python guy before the analysis has even started (i.e., most of the work). I know both really well. I use Python when I want to "get (semi) serious" production wise (I qualify with "semi" because if you're really serious about production, you're probably going to go to Scala).
But when it comes to taking a big chunk of untidy data and bashing it around till it's clean and cube-shaped, will parse, and has no no obvious errors, R is miles ahead of Python. R is where you do your discovering. Python can do it too, but I would estimate the cognitive overhead as double.
By the way, that's why people who "think time series" all day long (i.e., vectors, not objects), and who want to implement their algos, not think CS, will first typically build it in R, which is why CRAN beats Python all the time and every time for off-the-shelf data analysis packages. Data people go to R, computer-people go to Python (schematizing).
R is slow. That's its main problem. And that's saying something when comparing it to Python! But the gem of vector-everything makes it a much more satisfying language than imperative, OO, Python, when it comes to the world of data first, code second.
Finally I'd add that Python 3.x is arguably distancing itself from the pragmatism which data science requires, and 2.x provided, towards a world of CS purity. It's not moving in a direction which is data science friendly. It's moving towards a world of competition with Golang and Javascript, and Java itself.