East River was closer than Hudson River (where he ditched finally). There is also better rescue service close to an airport.
I doubt very much that any "rescue service" at LGA would have been any match for Vince Lombardi and his fellow skippers at NY Waterway.
Also, Dani, you seem so sure he could have made it back to the runway, yet just a few posts previous someone was saying that sim tests showed pilots making it back 50% of the time.