Tom, I will be short, because I think there is not much to say, Tamas
already wrote what he thinks is relevant about the Python interface
and I don't know any better than him.
Personally I think that if you want to do math and you know R fairly
well, then there is no point in switching to Python. Both R and Python
are nice languages with extensive sets of third party packages. The
main difference I see is that the R functionality is shifted towards
math and statistics. What Python offers is a more diverse set of
extensions (e.g. if you want your program to talk to web-servers
directly, then Python is a better choice) and a somewhat cleaner
language (IMHO). If you want to create sophisticated figures, then R
has an advantage in this, I believe.