In general, it depends whether you want to spot, or photograph. At EWR or in its immediate vacinity, you will likely get major grief if you’re seen with a camera. I’m not sure how easy EWR is for just spotting, as I have no interest in spotting myself. At JFK, it is possible to spot discretely by riding the airtrain around the terminals, but its no good for photography. For photography at JFK, there are a few places in the terminals but you’re likely to get questioned there too. For photography you need a car so you can get out to places like Howard Beach (don’t think the air train takes you to the right place). Having said all of that, LGA tends to be fairly friendly – there’s some big windows in one of the terminals and as long as you call the airport police when you arrive to tell them what you’re doing, they let you alone. Likewise, a walk from the terminals alongside Grand Central Parkway either to Plane View Park or the other way for Expressway Visual arrivals will yield some fairly nice pics with little hassle likely.

As suggested above, take a look at nycaviation.com for more detailed information.

Andy