Each organization is going to be different.

If you have a team that already rocks on PHP, migrating people to Python is likely to be a problem.

Depending on where you are located, finding good Python programmers could be difficult (stats: over 10x more PHP programmers than Python.

If your project is pretty far along, translating to Python might carry with it a non-trivial cost.

In general terms I think it might be easier/cheaper to launch an MVP in PHP with a sensible framework (like Yii) and then migrate to Python + web2py or Django for the "real" product.

I prefer Python, but it is hard to ignore job market statistics when considering what it might take to put together a team.

I'll go out on a limb and piss off a bunch of people in the process. I'll venture the thought that most PHP-only programmers are really bad programmers. Someone not formally educated in CS or having no experience in a variety of languages (and paradigms) before adopting PHP will probably be "challenged" as a programmer in more than one way. I have a feeling that, because of the nature of the language, a larger percentage of Python programmers are really good "traditional" programmers with a good balance between theoretical and practical knowledge.

Let the flaming begin.