Hey OP, let me help you out here. I am a EMT and I got my certification when I was a senior in High School. It was through a dual enrollment program so it was paid for by my HS through a local community college.
I forget exactly how long the program was, I think it was around eight months. I know that I received 11 college credits for going through it. I think that getting your EMT vs. CNA depends on several factors. There are pro/cons to each.
You're young and if you can find a job right after you are done with your EMT certification, then great. However, like others said, the job market is saturated. I just looked for a friend yesterday, and there are NO jobs in the ED as a tech in the entire state! A good thing about EMT certification is that if it is done through a community college, then those credits hours will be included into your GPA when applying to medical school. I had ~16 because of the several refresher courses I had to take to keep my certification current in my state. All 16 credits were an "A" so it would definitely help with the GPA. In my state, CNA courses are shorter, but they are not typically offered through a community college. But, if price is an issue, then a CNA course are typically less expensive because they are not offered through the CC and are shorter in length.
In my state, you can get a tech job in the ED whether your have your CNA or EMT. There is no preference to which you hold. However, like I said, the job market is extremely saturated. With your CNA, you can work on the "floor" or in a nursing home too as an NA. You can not typically do this with an EMT, they usually require you to have a CNA license. Working in a nursing home is not the most ideal situation, but it is clinical experience. I would investigate what type of certification your local ED's prefer their techs to have.
Since I completed my EMT program, I have been working in the ED as a tech (~4 years). I was very fortunate to get in, especially because I did not have any experience. At my current job, I help do the hiring in the ED for techs and can tell you when a new job is posted, we received >60 applications. Most of these people have a good amount of experience. Those who are hired, typically have >5 year in the ED. I would suggest that no matter what you decide to do, when doing your clinical rotation(s), make friends and get some contact information. It always helps to know someone on the inside as these jobs are very competitive to gain. I have have the opportunity to obtain thousands and thousands of clinical hours in the ED which has opened a lot of doors for me (research in the hospital, a publication, and leadership exp).
The bottom line is that if you can get a job with your certification, then do it. If not (which is likely), then there are many other ways to gain clinical experience (as a scribe, shadowing, volunteering, etc) that may be less work and not as expensive. I wish you the best luck, let me know if you have any other questions!