Pre-concussion Bergeron was a lot better, but now its very close. I would go with Bergeron because hes producing just about equally on a very offensively challenged Bruins and because he's a lot better in the face off circle. Last year, I would have gone with Zajac easily.
I think Bergeron is the better player overall because of his greater offensive potential but if you are asking who is the better at playing both ends of the ice, got to go with Zajac.
I also watch a lot more Devils hockey, especially these days. My memories of Bergeron are predominated by his rise to prominence as a rookie, his injuries, and his taking backseat to Savard, Chara, etc. over the past two years.