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.
Yup you are right. But python is a bit on the slower side in comparison to PHP and that is why the dilemma.
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.