Python + Django or PHP + Zend Framework
There is no direct one-to-one step or howto available for migrating PHP+Zend to Python+Django as both are different, different languages and different design models. So, migration is not easy.  For other comparison questions you have asked, the answer is it depends upon the use case. But just in case you are comparing for the project which you are going to undertake, go ahead with the one you are more familiar with and if you planning to start, then the answer will depend upon the camp you are asking. PHP camp will say start with PHP and it's framework and Python camp will say start with Python and Django. So, this is subjective too.
when building a web 2.0 application, which one is superior in terms of

performance
availability of features 
documentation
ease of use

Python + Django combination or PHP + Zend combination?
Also how easy it is to migrate one to another, so if I have written stuff for PHP + Zend combination how easy is it to move them to the Python + Django option?