The key difference concerns medical repatriation:
Medjet covers hospital to hospital transfers; once you are medically stable, you choose the hospital that you want to be transferred to. They cover the transfer if you are more than 150 miles from home. Medjet is a membership program (not an insurance carrier).
Most travel insurance providers decide where you will be treated. If the policy includes medical repatriation, they decide when you will be returned home. Read your policy carefully. There is a difference between medical evacuation (nearest adequate hospital) and medical repatriation (to a hospital in your home country).