It depends on where you work as a CNA. If you become a CNA and work at a nursing home, or a similar long term care facility, you will not gain much. Duties are basically tons of residents, with a standard day to day routine of meals, changing diapers, bathing, meals, changing diapers, bedtime.
If you can get a position inside of a hospital, you'll learn more. I'm taking my pre-reqs starting this fall, to one day get into an RN program and I decided to get a job as a CNA. I knew immediately that I didn't want to work in a nursing home. I landed a position as a CNA in a large hospital on the MedSurg unit, and was told I'll be floated to emergency room periodically, and eventually to many other floors. In a setting like this, I know I will see much more, and in turn gain much more experience and insight than I would at a nursing home.
So if you decide to go the CNA route, it's a matter of where you work that will offer you more or less experience.