Both are valuable. I use both doctest and nose taking the place of unittest. I use doctest for cases where the test is giving an example of usage that is actually useful as documentation. Generally I don’t make these tests comprehensive, aiming solely for informative. I’m effectively using doctest in reverse: not to test my code is correct based on my doctest, but to check that my documentation is correct based on the code.
The reason is that I find comprehensive doctests will clutter your documentation far too much, so you will either end up with either unusable docstrings, or incomplete testing.
For actually testing the code, the goal is to thoroughly test every case, rather than illustrate what is does by example, which is a different goal which I think is better met by other frameworks.