You can use subtests to build parameterized tests, you can't use
parameterized tests to build subtests. The standard library can also
be converted to using subtests *far* more readily than it could be
converted to parameterized tests. There's also the fact that creating
a decent parameterized tests without subtests support requires PEP
422.

If you're telling us we can only have one, then I choose subtests, and
third party test frameworks can layer parameterized tests on top.
However, I think you're making a mistaking by seeing them as
*competing* APIs, rather than seeing subtests as a superior
implementation strategy for the possible later introduction of a
higher level parameterized tests API.