Seriously, look carefully at a street map and transit maps. Somerville and Cambridge are wide cities from northwest to southeast and are basically stacked on top of one another. Proximity really depends on location. Somerville is a great city, and you should definitely include it in your living options as long as the location works for commuting. Similarly, just being in Cambridge doesn't ensure an easy commute. Traversing east-west across Somerville and/or Cambridge is a laborious, sucky commute that I would seek to avoid, if possible.