hii..
I will sumerize grossly..
TENS : excellent for nerve related problems involving pain, in radiating pain it works great. Works on Pain GATE theory. Can be used for muscle re-education also, it depends on one'e chioce and assesment.
LASER : Used for tissue healing purpose, again use depends on assesment, but as far as my knowledge goes it has nothing to do with pain/ PAIN GATE mechanism.
Thanks