I did the ER to ICU route. I joined a new grad program which was 1 year in the ER, combination of hands-on and classes. I then transition to the ICU. I think the ER gives you valuable experience with drips, and vents. It helps you to think on your feet and react appropriately based on what you are being presented with. As TLLCRNA said, you learn to handle unstable patients and how to multitask. Many times you really are starting from scratch. A patient might come into the ER in serious distress and needs to buy a tube quick, but there is no access for drugs to be given. I think this environment prepares you later on to react to "emergencies" on the floor and codes etc. Going to the unit will be a substantially different sphere of experience. The focus is completely different. You will appreciate the training in the ER as it will show you how disease progress. Normally we don't get to see what happens long-term in the ER with the patients as we are more concerned with the stabilization and hopefully a quick transfer to the unit that will help the patient best according to their needs.
If you really want to get the ER experience to add that to your belt, then by all means go for it. It is a very marketable asset, especially depending on the hospital/type of ER it is. If all you really want to do ICU though, and that is really where your heart is, you can always just for that and then try to cross-train in the ER to get that experience too.
Hope that helps.