I second a vote for the MAT. Although I have not taken the GRE, I did however just take the MAT a few days ago. While the test was hard it was only a hundred questions and takes 50 minutes to take. The test was also inexpensive, 65.00. However, I strongly urge you to at least do some practice test, since the testing format is different from any kind of test that I ever took. You can buy a practice book for less than 20.00.
Now, in regards to the best nursing experience for your resume. I feel you will probably put yourself at a disadvantage if you stay in the ER. I have been a nurse for 4 1/2 years and have worked in the SICU, ER, and now in a MICU. ALL have been in a level one trauma hosp. and in an academic setting. I spent most of that time in a the ER, and we even had a 7 bed MICU annex within the ER, however, I only spent a small portion of the time in that area, we constantly got rotated throughout the ER. In addition, that ER was one of the busiest in the nation and very well known. Having said all of that, it is still a tough sell when explaining all this to an anesth. school, when trying to convince them that my ER experience is as good as an ICU. SO my point is, it would probably be best to get into a good ICU, I prefer The ER to the ICU, but I also want to go to anesth. school. Therefore, I went back into the ICU and by the time I hope to statrt ANesth. school next fall, I will have 2 yrs ICU and 3 yrs ER experience. I feel I learned to think on my feet and in a critical way in the ER, but I did not get enough vent. and vaso-active drug tit. experience, not to mention zero swan-ganz exposure in the ER, as I do in the ICU. In addition, since you are a new nurse you don't want another thing such as only having ER experience to weaken your resume.
Good luck with your journey.