You will never go wrong if you start your project with django(Python framework)...it has been so active in the recent years and promises to be the future because even the python language itself is so active.
The difference in speed between Python and PHP should not be a deciding factor in choosing between the two web frameworks. Many of the core PHP functions are implemented in C directly, so they are very fast, but once you add a framework into the mix, speed drops off considerably because it seems the PHP interpreter is not so good.