Gosh that was the same question I asked myself when I was in school. When I was in nursing school I was an extern for a year in a large inner city hospital. My last semester of nursing school I precepted in a CVICU. As a nurse right out of school I worked in an MICU for 18 months and now I just transferred right back into the ER as a nurse. I might be able to help you distinguish the differences.
In the ICU, you will concern yourself with whole body systems. If you work in a CVICU, Neruo, or any kind, you might have a focused specialty, but you will concern yourself with the presentation of the disease process on the body as a whole. You will learn patho very well because the multiple co-morbidities will make you. If a person has liver failure, what will you see? How will the labs look? What will I do for pain? How can I sedate? Are they vented? Do their lungs comply? Why are their peak pressures up? Are they septic? What does the central line tell you? How do the pressers effect the body?
It's a wonderful learning experience because seeing the body as a whole and the long term effects of disease conditions will be invaluable to you.
Now that I am in the ER, it is a way more focused assessment. Patient can't breath? Make them breath. Patient is dehydrated? Get them fluids. Pt has a gunshot? Where is it and what is it doing to the body. Patient needs blood? Get them blood. The ER is way more direct and I feel it helps you become almost as a "jack of all trades" because you don't get great at one thing, you get good at a lot of smaller things. It's wonderful for somebody that likes to see a lot of different things, such as myself.
You will get a great experience anywhere you go. ICU and ER are two different things, but each has their benefit. I like them both and continue to work overtime in the ICU where I was at, while still working in the ER. I guess right now I like ER because of the acute immediacy of the situation. I like getting patients stabilized and moving them on when they are. The ICU I sometimes would have the same patients for months.
Take every experience and learn from it, no matter what you chose. Good luck to you and I hope this may have helped even but a little.