I have done both and prefer CICU which is where I work now. While there is a MAJOR focus on the heart you also have to remember that the heart effects every organ system, and many kids with congenital heart defects may have a syndrome of defects which affects multiple organ systems. We have kids with kidney, respiratory, liver, bowel, neuro, endocrine, oncology issues and hosts of others. Just an example I had two patients today, one is a premature neonate with a prenatally diagnosed congenital heart disease, now that the baby is out we actually don't think there is a heart defect at all but a brain MRI showed severe abnormalities. The parents are now facing decisions related to quality of life and palliative care - completely unrelated to a heart issue. The other kiddo is a few months old with a corrected complex heart defect, unfortunately this patient has a host of complications and a syndrome. Active issues aren't even related to the heart but to inability to wean from the vent (trach dependent), inability to advance feeds, dialysis dependence and new seizures - none of those things are cardiac but are all issues she has and that we deal with in the cardiac ICU.
Some kids are born with their problems and others acquire them later in life, we'll get older kids who end up with post chemotherapy induced cardiomyopathy and other diagnosis that can turn up later. Yes most of the patients are babies but we get all ages, in one day you can be taking care of a two day old and a 56 year old (yes I had that day) at the same time.
Generally speaking I think the patients in the CICU are sicker but it's because they have such little reserve to begin with. I worked in the NICU (Level IV) for two years and I will tell anyone that the babies up there can be sick as snot, especially the term PPHN/CDH kids and the preemies who get NEC and spiral down fast however looking at the units as a whole the cardiac kids are much less trustworthy. My first cardiac code was a three year old who spent the morning bouncing on his bed watching mickey mouse and waiting for a bed on the floor to open up. One moment he was fine next we were coding him, cracking his chest and putting him on ecmo. Sadly he didn't survive. Their hearts are so tenuious, one point drop in a base deficit or a half point rise in a potassium level can completely knock them off the edge and into a deadly spiral. Many, MANY of the babies have their chests left open after surgery so consider that too, is that something you feel like you'll be able to handle? Literally watching your patients heart beating with the possibility of needing to use three fingers to do chest compressions on it?
I don't think you can say the units are similar at all personally. I think the transition was easy enough though there were a few things I struggled with initially but I think any ICU to ICU transition shouldn't be all that hard, so long as you know how to code a patient technically you should be safe in any of them but to say they are similar I don't think that's true.