TENS vs. E-Stim Boxes

Can someone explain in very subjective terms the difference between inexpensive readily available TENS power units vs. the comparatively expensive E-Stim boxes that are designed for erotic play. I see enthusiast who LOVE their expensive boxes, but technically what is the difference in the two classes of power supplies?

PRICE!!!

OK So what is the best deal on a TENS unit?

TENS 7000 (about $32 on Amazon.)

The main difference is the stimulation patterns and frequencies. TENS are designed to alleviate pain with some having EMS features (Electronic Muscle Stimulation) but while they offer stimulation,it is very different to a specific designed box.