I'm not going to argue against seating or passenger capacity differences between the different types of rail cars because there are. But, I sincerely believe the DMUs should have lower fuel consumption and therefore emit less pollution. In a "green" state like California, pollution is a major issue.
I've read much about the efficiency of push-pull vs DMU trains, and most commenters suggest push-pull trains become more efficient at 4 cars - that at 3 cars or less DMUs are more efficient. I'm not qualified to state if that's true or not. Because the size of San Rafael city blocks downtown, push-pull trains will be limited to 2 cars, whether they're single of double deckers.
Not every commuter train in America uses double decker cars. I see nothing wrong with SMART using single deck cars initially. Maybe when it's time to add more cars to the SMART fleet double decker cars can be bought then.
I realize people look at data, especially subsidy per rider, closely. But when the train agency is losing money on every rider, that's not what counts. Subsidy per rider data is used to determine what type of system to run, and how much of it. But politically, they're expected to provide trains at a certain frequency so many hours a day. The cheaper they run those trains, the lower their operating costs are. How many engineers and conductors they need is set by the number of trains they run. The only other variable to reduce costs is to increase the fuel efficiency of their trains.
Short trains are apparently best ran with DMUs.