That really is a tough one. The question to ask yourself is, once you are officially on your own, will you have the support of the other nurses? Is the teamwork in the department (not just with you, but what you observe around you between existing staff) there? Is it strong?
I have only been an RN for 2 years, and went straight into the ED. It *is* a tough way to go, but I have been blessed to work with a VERY strong team which is always there for each other. IMHO, there is No Way to jump into the ED without strong teamwork. Once you have a critical patient, especially if it is a type of case you have little to no experience with, pt safety is paramount. Hell, the paperwork alone can be daunting.
With my coworkers, we hear of a high-acuity ambulance arriving we drop what we are doing whenever possible and jump in to help out until we know the pt and primary nurse are settled. Its just that simple. If your coworkers do this, you are set. If they do not, you may want to finish your residency and then start looking for another ED.
Best of luck to you, I hope it goes well!