JFK is actually even easier, in that you have LIRR trains departing from Jamaica to Penn Station (and in the future, Grand Central) every few minutes at nearly all hours of the day, and the transfer is a single elevator ride. You can be in Manhattan within 35 minutes of leaving the terminal.
JFK: just have to transfer from AirTrain to regular subway in Jamaica (?) Queens.
Newark: take the bus to Newark station and take PATH train to lower Manhattan.
Plus you get to see real New Jersians and New Yorkers. They are not overly friendly and they dress way too fancy compared to the Californians.