They aren't mutually exclusive. An always-available self-driving car service would drastically help the usability of subways or commuter rail in the crucial last 2-3 miles. If I could hail a car to drive me the 2 miles to the Caltrain stop and then hail another one to drive me the 3-4 miles from 22nd St. to the Sunset, a much greater percentage of the city becomes available to me without a highway trip.