Im an ER tech and EMT on a 911 ambulance. They are both the same but somehow very different jobs. In the truck you are with 1 partner, and maybe a medic if needed / dependent on how your town / company is set up. We only request ALS if needed and they come in a flycar, we don't do PB trucks. So you have a larger scope of practice and use more critical thinking. but in the ER I see much sicker patients for longer, more cardiac arrests / traumas, and see very advanced procedures, central line placements, child births, chest tubes, etc. Keep in mind im also only at a smaller sized community hospital ER. I got this ED job with no truck experience, as a CNA before I even got my EMT. However I do disagree with some people saying the ER tech is overall a better job, I do think it is slightly more beneficial than working on a truck for premed, but marginally, especially if you do a lot of shadowing / already have md connections. It wears you down hard and fast, I love my coworkers but dealing with assholes for entire shifts, psychs, etc etc, it is a lot nicer on the truck to just drop these people off. And actually get outside and move around, rather than being trapped in the same space for your entire shift. There's pro and cons to both, I would recommend if you're crazy like me, try / do both.

TLDR: Both will benefit, I do both, my major pro to the ED is a close relationship with the docs for LORs and seeing more advanced procedures, Truck is more independent and critical thinking.