The question is quite clear, ... note however that I am NOT asking about a feature comparison (there are a lot of them already), nor am I asking about which one you prefer !
I have myself a clear preference for doctests, I use them for everything, even if those are not to be used for documentation. But what I am wondering is : is there anything you can do with unitests that you cannot do with doctests ???
Some tests will need things like databases set up and initialised. 
This could make doctests:

very verbose (and therefore not good
documentation); and
probably inefficient because in doctests you would typically set 
up the database for each function or class. In comparison, 
unit tests more easily could use the
same database to test many functions
or classes.
Python: Does 'unittest' have something that 'doctest' hasn't?