Well if you are looking for a full credentialing in all aspects of nursing CCU is the way to go. What you should get credentialed for in CCU includes, Open Heart Recovery, IABP (balloon pump), CCVHD/CRRT (continuous dialysis), Ventillator/Trach/Intubation care plus all the thing that you would lfind in any other unit IV, NG, Central line and general practice nursing. The ER/ED is known for the triage and instant adrenaline rush things happen fast and furiously especially in a Level 1 trauma center, but the ER/ED you either treat and street or admit and them they are gone. In a CCU will see some Pt's from arrival to transfer or death depending on severity. You will also learn how to deal and comapassionatly treat families and loved one who have just lost someone, which I am sure happens in ER/ED too but probably you won't have had any real time to create any relationship withnthe Pt or family as you will in the CCU. Anyway you also do not have to pigeon hole yourself to one thing you can work CCU for a while see if it is your thing and if not jump back to ER/ED which it seems like is probably your comfort zone seeing your current credentials.
Good Luck whatever you do, stick with it long enough to find out what you llike.