There are two LGA approaches with good views, both from the south. Which one you get depends on a wide range of factors, including winds and traffic. IME, there is no predictable pattern as has been suggested (and I watch this every day from my office window).

The EXPRESSWAY VISUAL approach to rwy 31 goes up the East River over Brooklyn, so seats on the left will have decent views of the city and seats on the right will have views of the field at JFK.

The RIVER VISUAL approach to rwy 13 goes up the Hudson, much closer to Manhattan, and seats on the right will give you excellent views. Looking right down into Time Square lit up at night is pretty cool.

None of the approaches to JFK will have very good views of Manhattan due to the distance, except perhaps an approach to rwy 13 heading north over Brooklyn before turning east (used infrequently - saw it yesterday for the first time in recent memory).

You can get charts for the LGA (and other) approaches (IAP) at FlightAware.com here. You can also see which runways are in use, for example, here.