If I were you, I'd let the issue drop. Since you asked to go to an alternate city, your compensation would've been smaller - if anything at all - anyway... if I'm not mistaken, according to the (CoC, DOT, whoever - I don't really know), the rule is that if UA can get you to your destination within something like an hour, you're not owed anything. So if UA put you on a flight, to say EWR instead of LGA, and gave you cash for transportation to LGA, you wouldn't be owed anything.