E.Z. Flyer: I think the answer to your question is that up until the 3:28 mark (1600 ft) the crew were planning a return-to-airport at LGA. At that point they had a clear view of the airport and decided it was too risky. A 2-D map does not show vertical obstructions, which were, of course, not a factor on the big wet runway called the Hudson.
The pilots looked at all the factors and decided the Hudson was the preferable option.
One other point that was brought home to me in simulating this event (and the simulation actually tracked the real world better than I expected) was that even if one could make either LGA or TEB, one did so with no reverse thrust available.
I'll leave it to the real A-320 drivers to calculate how much runway they'd need with no reversers and a touchdown in maximum-glide configuration/speed.