I think this may help you: Is the R language suitable for Big DataIs the R language suitable for Big Data
I think it really depends on what you are comfortable with and what your objectives are. In Python, I can execute SQL queries directly into Pandas data frames. From there, data cleaning and visualizing (I like seaborn) is fairly straightforward. I can easily perform matrix manipulations using Numpy, and the sklearn is a well documented ML package.
That being said, I still like R and there are definitely times when I will use R. Learn both!