I have MegaSceneryEarth for LI and nothern NJ.  While I think that their scenery get expensive if you purchase a lot the scenery looks fantastic as long as you have some altitude.  I do not have either ManhattanX or JFK, I have read good reviews of both, but I have also read that both are FPS killers.

If you like flying around NYC and LI you could get all three.  In your scenery library you would need to have JFK at the top, below that ManhattanX and then below that MSE.  If you add any aiports in the MSE coverage area you will need to put this above MSE in you scenery library.

Rod