Depends on where you're going from, but the LIRR is rarely appreciably faster: express subway + AirTrain, vs. LIRR + AirTrain, both take around 60-75 minutes to

Depends on where you're going from, but the LIRR is rarely appreciably faster: express subway + AirTrain, vs. LIRR + AirTrain, both take around 60-75 minutes total from most parts of Manhattan. The only real exception is that the LIRR is faster if your starting point is right next to Penn Station; if you have to take a subway to get to the LIRR first you lose most of the advantage vs. just taking the subway the whole way. (Also, the LIRR is both less frequent and more expensive.)