Disclaimer: I am not a pilot (clearly), I am not even a real SLF
That said, I think part of the decision making process and something that is not being discussed now, may have been mitigation of negative outcome. LaGuardia airport is located in a densely populated area. An unsuccessful return would have been a disaster not only for the passengers but potentially for a large amount of people on the ground. Going into the river avoids at least significant collateral damage, if things go badly.
As far as East River goes, if you know New York, you would notice that East River is less than half the width of Hudson, is crossed by no less than 5 bridges and has a big island in the middle (so really there are two rivers right at the same lattitude where the plane eventually landed). The portion of East River near LGA that turns to the east (Hells Gate really) is even smaller and completely unsuitable for landing (barely suitable for navigation, which I know having sailed there a time or two
). So, East River would have been a terrible choice for ditching and almost guaranteed failure.