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 ???
There are some test scenarios doctests simply don't cover very well. That's OK since, as Lie pointed out, doctests aren't meant to be a comprehensive testing solution - they're meant to ensure that simple interactive-prompt style examples in your documentation (including docstrings) don't get out of date.
Writing actual unit tests, on the other hand, allows you to unlimber the full power of Python in deciding how to compose your test suite (e.g. using inheritance to share not only test set up and tear down operations, but also actual test methods).
doctests may be a part of that, but they aren't a complete testing solution (except for small, relatively self-contained operations).
It's probably worth browsing Python's own test suite (the test package) and taking a look at some of the tests in there. While doctests play their part, most of it is written using unittest.
Python: Does 'unittest' have something that 'doctest' hasn't?