My PICU has a combination of both, so I have some experience with both areas.
I wouldn't stick with the assumption that all patients in the CVICU are babies or little kids. It might seem like most of your patients are young, because many congenital heart defects are diagnosed and corrected in infancy. However, (if your unit does open heart surgeries) you'll be taking care of patients of all ages as well. Some congenital defects aren't repaired until late childhood, and other defects require multi-stage surgeries. The later surgeries might be done as teenagers or young adults. Likewise, complications (such as a non-functioning replacement valve) might require these patients to have additional surgeries later in life. Since management of a patient post-op for a congenital defect is different than management of, say, a CABG, our unit also takes care of adults who are status post these surgeries. It's not uncommon for a 40+ year old to come to our unit post-op.
You'll also see patients with arrhythmias, syncope, s/p cardiac catheterizations, surgical complications like infection and chylothorax, possibly patients awaiting transplant, having symptoms of rejection, or multiple other issues. I don't think it will pigeon hole you at all. Cardiac problems in children is one of the most diverse fields there is. You'll have lots of other options- PICU, general peds, peds cardiac floors, adult cardiac ICU, telemetry, etc.
Like any peds field, there is no answer to the question "What's a typical day like?" You can be caring for one or more of kids that I described above. Some might be immediately post-op, intubated, with chest tubes, on pressers, very, very sick. Another day you might have a much more stable patient ready for transfer to a lower level of care. Absolutely no way to tell. But you can bet you'll be doing a lot of monitoring of vital signs, heart/lung sounds, EKGs, fluid balance, assisting with procedures such as chest tube and pacing wire removal, echocardiograms, abulating patients, teaching, communicating with cardiologists and much, much more.
There's also really no answer to which patients are more critical. It just depends. General PICU patients have a huge variety of illnesses: from trauma, to neuro, respiratory, GI, kidney, sepsis, poisoning, cancer and blood disorders, ex-preemies with chronic medical issues, status post operation, and genetic diseases. Depending on the acuity of the PICU and what children happen to be on unit at the time, PICU patients can be more critical than those in the CVICU. Acuity of patients seems to cycle. We'll have a few weeks where the kids are really sick, and then a few weeks when they are less critical. If your CVICU is doing a lot of surgeries, then the kids in CVICU will probably be more critical, in general, then the PICU, but fortunately they recover pretty quickly.
Where are the codes happening? Hopefully no where. I'm hoping that you're not going to choose your placement based on where more children are dying. Codes happen in both units- but fortunately not routinely. As PICU nurses, we pride ourselves on being able to recognize decompensation in our patients and intervene before it becomes a code situation. In the year that I've worked at my current PICU, we've have far more general PICU patients code then those with cardiac issues. But if you're asking which patients are going to experience more arrhythmias and require PALS protocol, then those are probably the CVICU kids.
Personally, I prefer general PICU kids. I like the variety of all the different diagnoses. I also have a passion for peds oncology and hospice. I don't have a ton of experience with the cardiac kids- especially those in the immediate post-op period, so I'm still a little nervous taking care the cardiac patients. It really just depends on your interests and your preference. Maybe you could ask to shadow in both units to get a better feel for the positions. I've learned that people often have a lot of pre-conceived ideas about pediatric units that turn out not to be true in the real world. I certainly wouldn't make your decision based on the ages of the patients you want to care for. Instead, try to find the area that you think your experience and interests would be the most useful.