My anecdotal estimation of the Northbay transit market has Marin with more ridership than Sonoma. This is based on what I have seen from riding GGT's #80 trunk bus line. And an important segment of that traffic is south of Larkspur, outside of the current version of SMART.
SMART wouild be smart to keep their doodlebugs as simple and as standardized as possible. Easier to lease or sell in a worst case scenario.