Consider Starwood. Get 25K for credit card signup bonus. Sheraton LGA East is right in Flushing Chinatown. Tons of good, cheap restaurants within walking distance. Nice and clean, better than Holiday Inn. Subway around the corner. You can take express train into NYC midtown during rush hours in about 35 min (and back in evening hours). 10K a night. And every 5th nights free. Or you can do Cash & Point there.

15 nights is a lot, esp in NYC. You most likely will have to stay outside of Manhattan.