I've had both. In LAS, I walked through, somehow set off the metal detector with nothing on me, was directed to walk back through and instead of a beep got "selected" for a random at which point I was directed to the porno-scanner (opted-out and got a cursory pat down, where they chose to ignore the fact that I was still wearing my belt).
In SFO, got a beep and they asked me for a handswipe. Thirty seconds later I was on my way.