I'm a medical assistant and my program was 6 months. But 6 months straight, Monday-Friday. I also was a CNA and that class was 2 weeks. As a CNA you are going to be doing the grueling dirty work. You learn a lot really fast on the floor and it really helps determine if you can make it as a nurse. As a medical assistant we help coordinated doctors offices, inventory, scheduling issues, rooming intakes, vitals, injections and stuff like that. Pay wise I make twice as much as a MA then I did as an CNA. I don't work weekends, evenings or holiday as I did as an CNA. Also as a CNA you most likely will work in a nursing home, which definitely has its own pros and cons.
Overall I think either one will give you a good perspective of the field, and I personally love my job and learn way more now then I did as a CNA