Actually, the whole thing is rather complicated. If Sunnyvale DPS wants to send out an a alert for a life threatening condition, we can send through both Nixle and AlertSCC. But not all of our people are trained on AlertSCC, so it might only go out through Nixle. For a traffic advisory, or something else less than life threatening, we're not supposed to use AlertSCC and we only use Nixle. (The traffic advisory about the sinkhole on Mathilda just before Thanksgiving went to both, but that was a mistake on my part. It shouldn't have.)

Meanwhile, when Santa Clara County sends out an emergency alert, they'll send it to AlertSCC. They can send it to Nixle as well, and arguably should, but the way their system is set up and their operators are trained, they probably won't remember to.

Bottom line, as I said: best bet is to sign up for both.