As someone who has done both I will say to you this.
Go to nursing school
anyway because either way once your body has had enough of EMS and the lifting etc you will have a back up plan. If you truly do not think nursing is for you pick another career, but always have a back up. Trust me, I am working with a bunch of medics who are now over 50 and who are scrambling to do nursing school, or some other career because they realize that they are getting older and less tolerant of the long shifts and physical labor. Even if you do nursing school you should be able to get away with only working full time as a new grad for a year or two before you can drop down to per-diem as a nurse and go back to the ambulance full time. That is what I did, I realized I wasn't quite ready to leave the truck yet. I wouldn't judge nursing based on working as an ER tech, unfortunately you guys tend to get a lot of the grunt work which may make it not so enjoyable. I will also say that working the ER was not for me, even though I was a medic. The patents can be less than pleasant when they are waiting for hours on end, and then the psych patients tend to get on every last nerve of my being, especially when they start feeding off of each other. Anyway, that is the good part about nursing is you can pick different areas to work in.
Fast forward to ten years later (now), I have been a medic for 14 years and worked as a basic for 6 years, when my back is really sore, my shoulder hurts... you get it. Now I am thankful I have a back up career that can get me off the ambulance. I will always do it part time or per-diem, but it's nice to have an alternative that is easier on the joints and body. I can also literally make more working as a nurse 32 hours a week than I can working 48 hours a week on the ambulance.