The TENS units use a different form of stimulation than the Electro-sex system offered here. We own both and I prefer my Electro-sex system while Sigel prefers his TENS unit. One of the major reasons I prefer mine is the fact that it is very difficult to actually hurt yourself with the electro-sex unit while with the TENS unit you can actually cause burns. The TENS unit provides direct to nerve stimulation where the electro sex unit provides stimulation to the muscles.
We are planning to test the cock ring portion of the kit tonight but Sigel is already skeptical that it will provide the same intensity as his TENS unit does...he likes direct stimulation to his prostate and that is harder to achieve with my unit.
My doctor prefers the electro sex unit offered here because there is less risk of injury, in his words it'd be pretty damn hard to actually hurt yourself with this unit. I've even pulled the leads off my body without the arching that can happen with a TENS unit.
It's a matter of how much intensity you are seeking, as far as I can see.