Of the two, I might suggest the EKG course if
the course is more than a simple "lead placement and STEMI recognition course. There's a LOT to being able to interpret the 12 lead and quite honestly, the average EM Physician is OK at it but it takes a Cardiologist or someone that's really
into studying the art of 12 lead interpretation to really
pull subtle details out of it. In a previous life I was a Paramedic and the module we had in class was more than a STEMI recognition course but it wasn't super in-depth either.
Notice I said I was (actually still am) a Paramedic. I did my EMT training 20 years ago and Paramedic shortly thereafter. I started nursing school
6 years ago. Did the experience of doing EMS for 7 years and having a very
public-interactive job help me with nursing school? Yes. Did it help me much after about 3rd semester? No. I got a lot out of having that experience but for the first couple of semesters but that simply allowed me to concentrate on the didactics and not have to worry as much about the clinical stuff. Most of my classmates had very little, if any, prior medical experience before starting nursing school.
Furthermore, I would actually recommend saving your money and not do either program/course. EMT isn't going to be all that helpful unless you actually work as an EMT during school and I wouldn't recommend that. You're not going to learn to apply stuff and make it useful to you in a relatively short ~150 hour program otherwise. I would highly suggest that if you are going to take one of those courses, wait until after you've had all the cardiology education in nursing school before starting the 12 lead course. You'll understand the material presented in that course far better at that time.
So... do neither for now. That's my advice.