If you stay mostly in SoCal, T-Mobile is great. If you go into more rural areas, NorCal, and especially Sierra mountain areas, then I'd stick with Verizon.
FWIW, the cheap $30 4G prepaid plan includes roaming. While spending time in Southern Utah, Northern Arizona, Nevada, I was able to roam, so I could call and text, but no Data until I hit major cities. I was really surprised how much I coverage I actually had in the middle of nowhere.