I've been in a similar situation for about a year.
A year ago I started to manage a team developing and maintaining a 10+ year old web site, which is programmed in PHP, which I did not have any previous experience with. I've used lots of languages (C, C++, Java, Python, Common Lisp, etc) and would have preferred developing with Python.
Existing legacy system built with PHP made the choice (continue using PHP) very easy.
I did not have any trouble picking up PHP, though it is a bit strange mix of low and high level concepts and seriously lacks harmony & elegance and "coherent vision" behind the language and implementation. And now I do understand why some call PHP "template engine gone berserk". Nevertheless PHP gets the job done. I seriously recommend using a good framework, such as Zend, which leverage PHP to a better (sane) language environment to work with.
Anyway, knowing or having experience with the implementation language, is not a requirement for project manager. Many PMs do great job without any programming experience, though it helps to have a programmer background (in general).
If you have no legacy and can start with clean slate, I guess both languages would do and it will be more a matter of personal taste and/or availability of resources.
Still... I can't escape the feeling, that Python would attract more competent programmers. Somehow the culture around PHP is less oriented towards "robust and elegant" solutions, and more likely to suggest a quick hack in "this works for me" habit.
To wrap up: PMs ability/experience should have nothing to do with picking the language. It's more about merits of the language (and culture) itself. And availability of resources.