as someone who worked both and an EMT and a CNA, I can say that I have seen more pre-med emts than cnas (I know of two other premed cnas).
EMT-basics (which correct me if I'm wrong, is what we're taking about here) are over-glorified taxi drivers to some poeple, getting an intermediate or medic gives you much more autonomy, and greater scope of practice as to what drugs you can push (this is where the fun starts), but all this changes from state to state. And it takes at least two years to reach these level in most states.
however as a cna (especially night shift, which I work, so of course I'm biased), gives you the chance to read charts, look at in depth medical histories, look up drugs, doses, what tests are ordered, what the labs are, and lots of vocabulary in Tabers.
Besides, my personal biased is that medical patients are so much more complex than traumas. The traumas I've had basically include getting an airway, a lot of duct tape, two large bore IVs with 500 to 1000 cc boluses, O's, and going 2.5 or 3 to the hospital. When traumas are dropped off at the ER, it's basically the same, get a airway (which they usually have if they present to the ER themselves), stop the bleeding, and a unit or two of PRBCs and 1000 NS bolus...maybe an amp of d50. Then ship them off to the OR (where I'm sure it gets much more complicated). But from the perspective of a tech or even a nurse, traumas are much more simple than med patients with multisystem failures.