If I recall various discussions correctly, the advantage with the electrics is potentially more power and better adhesion, allowing faster acceleration. MUs are the best, with all wheels powered, and also allow better dynamic/regenerative braking. But speed, as you note, is also an issue, and the faster the trains the farther apart they need to be. Caltrain isn't particularly fast (might reach 70 in a few spots) and has many stops, so it's somewhat BART-like. EMUs on Caltrans probably would allow more trains, just by accelerating better, but there's a limit to acceleration and braking rates given adhesion, optimizing power use, and passenger comfort/safety (without seat belts ;) of course). See: light rail, PCCs, BART.