Most definitely a CNA---it shows more character and work ethic than a phlebotomist. It shows that you are willing to get down and dirty and that you really care about the patients that you are working with. It also shows that you know HOW to work with people rather than just in a lab which is MUCH more valuable to your medical school application. Trust me...speaking from experience.
I do far more than just work in a lab. I have to be able to work with all sorts of different people, from the patients up on the floors, to the outpatients, to the med techs in the lab, to the nurses and doctors treating patients up on the floors. I have to be able to deal with people who HATE me, just because of what I do. As a CNA, at least you can say you're helping them immediately. I hurt people, and then they get the benefit when the doctor comes in and sees the lab results. I had to draw one guy the other day that sat there and complained about how many times he had been stuck since he got in the hospital, and how bad the phlebotomists were, and everything else. And then I turned around and had to talk to a nurse who was bothered because she didn't have her test results yet (but, to be fair, the doctor was upset with her first) because it took us a while to get blood from the patient.
So don't go on about how phlebs don't know how to work with people just because they're based in the lab.
The problem with finding phleb jobs is generally a lack of experience, after certification most people have around only 30 sticks. Finding a job at a plasma center is much easier and after a little experience a hospital job might be easier to land.
Depends where you get your certification done. By the time I was done with my class, I had 50 sticks and 20 finger sticks. I then did a clinical in an outpatient clinic in a military hospital, and probably was drawing 40 patients a day by the end of my 4 week clinical.
And the turnover rate is pretty high for phlebs. I know my supervisors were looking for 2 people to fill in the holes in our schedule and the lead phleb actually said that she'd almost rather hire new people because they are easier to train than the people who have experience.
OP, at the hospital I work at, CNAs do a lot of clerical work. They're the ones that put in orders into the computer and stuff. They also do the dirty work like bed baths and whatnot, but to a lesser degree, I think. As a phleb, I do some clerical work, but my job is largely running through the hospital trying to get the blood the techs need to run their tests.
I think regardless of what you do, though, it's not the job that matters so much as what you get out of it. As either a CNA or a phleb, you'll get a lot of patient contact and you'll have to learn good customer service skills, both of which are a plus for medical school applications and working in the medical field in general.
But I'm getting a CNA cert this summer because 1) that's what's available in my area both for jobs and the class and 2) it's like a month of night classes and it's really cheap. 
