I have a CMA (certified med asst) certificate. I attended school in the late 80's; an 18mos program. I strongly urge you to attend the 2 year program and get the AS degree. Employers will respect that more, making it possible to request a higher pay or better schedule. Additionally, many do go on to pursue a nursing degree. You will be ahead of the pack, having a CMA, experience and an AS science degree.
As an aside, I don't really feel it would be possible to learn all of the patient care, lab work, and back office procedures on line. I think that online is great for basics like English or History, but you may regret not having the solid core knowledge that comes with classroom work and the "guiniea pigging" of practicing on each other.
Good luck!