The AirTrain is a huge and foolish waste of money. It will make the trip LONGER from Manhattan, Brooklyn, etc., and you actually have to go past (farther east than) LGA to get it.
The Q70 bus takes about 10 minutes from the 74th St./Roosevelt Ave. subway - which is served by 5 different lines, including an express. So you can get to LGA in less than an hour via the E train and 70 bus from downtown. Maybe in under 45 minutes from midtown. The AirTrain will be less convenient and no faster.
LGA is not like almost all other airports in this country. It is very close to residential areas and roads, not off an exit of a highway in the middle of nowhere. Would you build an AirTrain to Chelsea Piers? It's basically the same distance from residences and commercial streets where buses run.