[probably]

Well explained by chimeracoder. Data-table centric operations are much more natural in R while sequential objects (lists, tuples, and strings) are quickly manipulated in Python (there are more string/regex methods there).

I am a heavy Python user, but when I use Numpy/Scipy I don't feel like I'm using Python much anymore so at that point I either switch to R (or Fortran)... though I'm quite optimistic that at some point the pandas DataFrame can become my default storage structure from which I can parse out R tasks through Rpy, SQLite, HDF5, or possibly Reddis.

matplotlib is very verbose though; I almost prefer Matlab's graphics model... though less so than R's basic and lattice graphics.