Jenny,
Either unit sounds good, but the ICU would be a better choice if you are thinking about a career in anesthesia. For one thing, almost every CRNA program requires a least a year of critical care experience for admission. After you've been in NICU for awhile, you might check to see if your hospital has a critical care float pool. Not only will you great experience, you will also be working with the CRNA's, anesthesiologists, surgeons, etc., who will be a good cource of letters of recommendation down the road. Also, if you spend some time in MICU / SICU, you will deal with challenging cardiax cases anyway; in fact, you will get everything you'd get in the step-down, and then some, because your cases wil be a lot more involved on any of the ICU's than they would be in a telemetry / step-down.
Good luck!

:hatparty:
David