How many users you have in your database is not relevant to your choice of language. That might guide your data design and choice of database system. What generates load on a front end web server is concurrent users -- requests per second.
There probably won't be any difference any way you go, though I don't know much about the full stack for running Python web apps. The first bottleneck you run into running a web app on large data sets is always disk IO -- once your database is too large to fit into memory -- and that is orders of magnitude slower than the time it takes to execute your language code. I run several significant sites on Symfony that handle hundreds of requests per second.
Here are some performance comparisons among PHP frameworks:
Ignore the Symfony 2.0 parts because that isn't production-ready yet, but Symfony 1.4 is in the tables too.