As someone who grew up in Mercer County, I would say Central Jersey is anything between New Brunswick to the North and Bordentown to the South.
As for the NY vs. Philadelphia fansheds, the dividing line generally follows the old province line divide which runs diagonally Northwest from around Tuckerton on the coast to around Phillipsburg on the Delaware. Back in the colonial era, NJ was briefly divided between PA & NY. To this day, the right side of that line orients itself more toward NYC while the left side of that line orients itself more to PHL, even if, as the crow flies, a place may actually be closer to the other city. That's why it's a little odd that the Phillies have a minor league team in Lakewood which is clearly in NY fan country.
And yes, Mets fans who lives in NJ much prefer to see their team here than at Citi Field. New Yorkers who moved to the suburbs have this odd mix of passionate love and utter disdain for the city. They love to say NYC is their town yet they refuse to step foot in it. When the Phillies win and pack the place like they did from 2007-11, the Jersey Mets fans are out of luck. Usually, however, when one team is up, the other is down...or both are down together. Both franchises posting winning seasons at the same time has only happened five times ('75, '86, '06, '07, '08). Only in '07 & '08 did the two teams find themselves in a penant race against each other. In that context, it's easy to see how one team's fans may take over the other's ballpark.
