Arguably the San Mateo area as between SF and SJ.
South OC as a transitional zone between LA and SD - Camp Pendleton being a dividing line.
I think it's difficult to argue for San Mateo, since the Bay Area is a single metropolitan area. There are not "San Jose suburbs" as opposed to "San Francisco suburbs" and there is ample commuter flow in both directions.
Camp Pendleton does represent a pretty hard line between San Diego and Los Angeles, which is striking when you drive along 5 and there's a vast empty space. I think it's more interesting to look at the 15 corridor here, since Escondido is clearly San Diego and Temecula is clearly Los Angeles and there are some people who live in between.
