I'm interested in black holes. Is is better to go into Space Science or Planetary Science?
If you are interested in black holes there are different career paths that you can take. In my opinion, Space Science and/or Planetary Science are not the best choices to study black holes as they may cover (if any) the theory and observations of BH only marginally. A best choice would be a career as a physicist, astronomer and/or astrophysicist.
As a physicist you'll have the mathematical tools and formalism to study the theory of blackhole radiation, the processes that creates and regulate BH growth and blackholes as a key component of galaxy formation.
As an astronomer you'll have the tools to study blackholes from an observational point of view while trying to match theory with observations. Blackholes radiate in a wide range of energies, from the radio to the gamma ray, therefore, indirect signatures of blackholes are suitable to observed with a wide range of observatories both on the ground and on space.
As an astrophysicist you will be between physics and astronomy, thus, an unique perspective into the theory and observations of black holes.
The main focus of my research is in super massive blackholes and how they can affect the evolution of galaxies. My university degrees are in Physics (BS, 2 x Ms and PhD) with an emphasis in astronomy/astrophysics (course requirements and thesis research). From my personal experience, I cannot stress enough the advantage that it is for my current research to have a degree in physics because it gave me the mathematical background and fundamental knowledge to tackle any problem.
If you are truly interested in black holes, the most appropriate degrees would space sciences, or physics with a focus on courses in astronomy and/or cosmology (the evolution of our universe) which are often offered as electives to those seeking a physics degree. A space science degree might get you the awareness and understanding of things like black holes, but to truly know them and to be able to mathematically describe them, theoretical physics and specifically cosmology (the study of the universe) is the the way to go. Sometimes this area of focus is classified as astrophysics. Become an astrophysicist like me! While I've never taken planetary science courses (just one environmental engineering course), I don't think that degree path would require topics on black holes. But that would be very appropriate if you were interested in the studies of planetary bodies and the evolution of planets of all sorts. I hope this helps!