Hard call!
When I was a new ICU nurse, I went to a seminar, the speaker was Terry Richmond, not as well known then, but now the author of several neuro nursing books. Anyway, she asked at the start of the neuro lecture, "How many of you here think the heart is the most important part of the body?" About half the room raised their hands. "You must be the CCU Nurses. How many of you think the brain is the most important organ?" About the other half of the room, including myself. "Ah, you are the ICU nurses," she said.
I did indeed think that, and I found it immensely challenging to work with head trauma pt's....but I will add, I worked with them in the acute phase. Once they are no longer acute, it can be, IMHO, a little depressing, especially if you are caring for a C2 fracture pt, who can't breath w/o a vent. I remember vividly, as a young nurse, with minimal life experience, a young beautiful black male, my age at the time, 22, who was on a rotorest bed, trached, knowing he was a living head for the rest of his life. I am ashamed to admit, I could not look him in the eyes, since whenever I did he would mouth the words, "please kill me." So, while I find neuro fascinating, know that there is a limit sometimes to the recovery these pt's can make, and I would never say that this kind of nursing does not have its own rewards, but each person has to know what they can and cannot handle deep inside. I prefer the ICU side of neuro, not the long-term, to me it can be too depressing. I know that there may be a neuro nurse or two who will tell you differently, and I am glad for that, b/c it takes all kinds of nurses, with different strengths and weaknesses to make the world complete.
I moved on from that ICU to a CT ICU. LOVED it!!! Still love it. Give me my cardiacs, esp my geriatric cardiacs, and I am in heaven!! From ICU to home care, I can't get enough cardiac. I feel it is very rewarding, b/c it is often me who determines the right weaning of IV drugs in the ICU, or picks up problems with their med combination in home health, and can usually see a dramatic improvement in a short time as a direct result of my efforts. (I admit it, I am totally into instant gratification!!

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So, if you want my TOTALLY biased opinion, go with cardiac. The HEART after all IS the most important organ of the body (as I now know!)
Good luck to you! PS, Post this on the neuro forum too, you will get cardiac junkies here! LOL!