Forecasts speak of rain tomorrow (3/20), so it's your call. Winds'll be from the East, so nothing spectacular will probably be happening at LGA
will opt for the 31s.
Javi, I would stay inside unless you'd be up for dealing with wet and miserable weather.
Tim, it all depends what airline you're flying out with. There should be photo ops from all three terminals, but only the Central Terminal Building offers these before security. There are two good locations--one by Pier D (American) and one by Pier A (Continetnal, et al). There will be huge windows and rocking chairs, but if you go up the escalator that's righ there, you'll be above all the action. Keep in mind that you'd be behind a window and it'll be raining.
If you plan on being there for any length of time, pick up one of the courtesy phones and dial the extension for the Port Authority Police Department. Tell them that you're an aviation enthusiast and just want to let them know you'd be taking photos of aircraft from 'the windows'. They may send a cop or to to get your name or check your ID
, but other than that, you should be in the clear.