In my opinion it would depend on what your goal is once you're out of school. If you're interested in cardiac nursing, I'd say shoot for CCU. For a more generalized experience, I'd say go for ICU or ER.
As for the difference between CCU and ICU, I'm not sure if this applies to all hospitals (or your hospital), but this is how it works with mine. CCU is where all open heart surgeries go, along with patients who have a cardiac or vascular diagnosis as their primary diagnosis. Sometimes a sepsis patient might end up over there if ICU is full. The ICU is more generalized at my hospital. We see traumas, respiratory failure, sepsis, DKA, overdose, liver failure, neuro patients, etc including pediatrics. This doesn't mean that we won't see people with cardiac problems, and lots of our patients are still on pressors and require close hemodynamic monitoring.
I might be biased since I work in the ICU of my hospital, but I feel like I'm seeing a wide variety of diagnoses. Not that you won't see variation in CCU, but at least at my hospital it seems as though you'd see a wider range of patients in ICU. You might not see a Swan in my ICU, but really the only place we use those is in the CCU here.
Hope that helps! Good luck