I commuted to Cambridge from Providence for five years, five days a week. It was terrible. Having to jump on the red line from South Station - or do the reverse later - really felt like a second commute on top of the commuter rail. Of course it was only 20 more minutes, but it just made the whole thing feel much worse. When I was job searching I did not look at anything that was not walking distance from South Station, Back Bay or Ruggles.

On the "home" side of the commute, I would not look in East Providence anywhere south of Rumford. That way you can cross the Seekonk River to the train without using 195/Washington bridge. The traffic on that stretch of 195 is terrible in both directions both morning and evening rush hours.

As others have said you can take the Red Line right from Quincy to South Boston and avoid South Station all together. But I would still avoid 195/Washington Bridge at all costs.