First of all, you have to determine which therapy is right for your specific needs. If you want to treat muscular spasms, poor blood circulation, or muscle atrophy, look for EMS units. If you want to focus on wound healing and non-muscular pain relief, then TENS units are the answer. The best electrotherapy machines are those that can provide both modalities. Any added ones are a welcome bonus. For instance, some units also provide interferential (IF) and microcurrent modes.