Go for thoracic ICU!! I work at Intermountain Medical Center in the heart & lung building just one floor up from thoracic, so I may be slightly partial! Its a gorgeous floor with brand new state-of-the-art everything, and being a level I trauma hospital, you will see the most critical of patients. You will be dealing with heart and lung transplants, LVADs (I'm sure they told you this already).
So, anyway I say definitely go for thoracic!