Here is Sully's own response taken directly from the NTSB flight crew interviews (links posted on previous page just before the CVR transcript).
Asked why he felt that the river was a superior landing choice, Captain Sullenberger stated that choosing to return to LGA was an irrevocable choice, and if he had made a turn toward LGA then realized he could not make it, he would have had no other options or enough altitude to return to the river. He was told that he cleared the George Washington Bridge by 900 feet. He stated before he would make the decision to land on a runway, he would need to be sure that he could make it without landing short or long, he could line up the flight path with the runway, he could stay on the runway, and that he would have a sink rate that was survivable and would not collapse the landing gear and create a post crash fire. He thought that being a little short could have been catastrophic for those on the airplane or those on the ground. He could not afford to make the wrong decision and he was confident that he could make a successful water landing.
Thoughts on choosing river or LGA into the context of Threat and Error Management: it was clear to me based on our position, altitude and airspeed and heading away from airport and time it required me to maintain control of airplane and analyze situation that a return to LGA was not possible. We were too far away, too low and too slow for TEB. Only other option that was long enough, smooth enough, wide enough was the river.