I'm using the SANTI BX400... I find it extremely warm. I have been diving every other weekend in Tobermory this winter and haven't been remotely cold. I wear a thin wicking layer under it.
Water temperature has been 34°F/1° C and dive times have mostly been in the 40 - 55 minute range.
The suit isn't inexpensive, but it's made well, with articulation panels in the joints and so on so it's very comfortable to wear.
I'm wearing it under a SANTI eMotion tri-lam suit.
The 400 would be far too warm in California I think, unless you are doing multiple long dives in a day. SANTI makes a 200 weight undergarment, and I think there is also a version with the core being 400 and the limbs being 200, which would be perfect I think.