That's why I love this site!!
I told my wife- let me ask the FGFers- any group of people who can argue passionately for 8 pages about original tires are going to have a lot of opinions about this.
I still haven't decided, but this info does help, so thanks to all (but no need to stop- keep it coming!!) I am particularly interested in the reasons why the concrete advocates are advocating it- is it more for structural strength or for waterproofing?

Bjorn, thanks very much for your input, and rest assured, experience does count! I will weigh all input based on that, and I greatly appreciate your experience and advice.

I have to admit, just from a layman's perspective, I've rarely seen concrete that didn't have some kind of crack somewhere, so perhaps my worries about cracking are moot, if both structures are prone to it, and the earthquakes are bound to come eventually. Sounds like I just have to be sure the exterior tar is done properly. I should reiterate that the ground here is very dry, with no snow, so I don't think the surrounding earth is going to get saturated with moisture.
In terms of structural strength- yes, the architect, structural engineer, and builder have all blessed the cinder block design, and the city is just about to issue the building permit. And yes, the codes in California are very strict- I believe every new structure has to be designed to withstand, if I recall, an 8.0 or greater quake.
Just some additional questions- should the rebar be forged or cast, and should I use 4 bolt mains when I attach the framing? ;-)