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 is a widespread misconception that doctest is for testing your code. doctest is intended for testing your documentation. doctest is intended to test that your documentation matches what the function/class/module is actually doing, and alerts you if sample code in your documentation becomes obsolete as the module evolves.
While doctest might reveal bugs in the code, it is not its primary purpose (e.g. like a change in code might unravel bugs in a unittest's testcase code, but testing the testcase code is not unitetest's primary purpose)

even if those are not to be used for documentation

docstring are automatically extracted out by help() function to become documentation for your 
function/class/module; you cannot make a docstring not a documentation. Users of your module/function/class (or you in a few days) might try to do help() on your function/class/module and get a surprise that the documentation is a bunch of codes.
Python: Does 'unittest' have something that 'doctest' hasn't?