AC Motors have higher EMFs. EMFs (electromagnetic field) are invisible lines of energy that surround an electrical device. Our bodies naturally create EMFs and when we are exposed to EMFs from unhealthy sources they interfere with the way our bodies naturally work. From causing sleep disturbances and headaches, to Fibromyalgia and Cancer, EMFs can do serious damage to our bodies. AC power alternates back and forth from the electrical source to the appliance about 60 times per second. It creates a weak electrical current that works on the appliance as well as our bodies. Many of the low end machines you will find in big box stores have AC motors.
DC Motors have lower EMFs. Direct current motors have a current that flows one way from the power source to the appliance. This flow of energy does not create that same electrical current and does not affect our bodies.
Although you may be able to get a cheaper machine with an AC motor, your primary goal is to make yourself healthier, so please keep the type of motor you are buying in mind when making this investment.