I know that it doesn't fall into your criteria of both coasts or Chicago, but I did a month in peds cards at Baylor in Houston and had a blast. It was an outpatient rotation, so I was in clinic Monday-Friday with no call and no weekends, but I also had opportunities to go into the cath lab and echo lab as well. Attendings were great, lots of teaching done during my time there, and tons of variety in the clinics with at least 3 attendings in clinic each day, I got to come in every day and pick which attending I wanted to work with. In terms of the East Coast, I've heard that Columbia in NYC is pretty strong in cardiology, and if you're looking for something smaller, MUSC in Charleston, SC (my home school) has a really good cardiology program for a mid-sized residency program. Also, the rotation offered at MUSC is an acting internship on their cardiology inpatient unit, so you'd get to see a bit more postop care.
One last thing, if you want a broad overview of cardiology and want to learn more about identifying different murmurs and things that will help in all areas of peds, I'd encourage you to do a rotation in cardiology that is more outpatient based. If you're interested in cardiology as a possible career choice and want to see the hospital side of things, the inpatient route is the way to go. Make sure you know which type of rotation you're applying for before you select where you want to go. Good luck with your search!