For NJ, the rule of thumb is to find and research what the best defense round you can use, and then do exactly the opposite. 9 times out of 10 you will be legal if you think that way.
My best suggestion is to travel west into PA and use whatever defense rounds you want, and whatever size magazine you want. Hell, carry out in the open and don't get a license; it's legal there too. So my question, what makes that line of separation between PA and NJ so special?
As far as what is the best defense round that is legal in NJ? I don't believe a defense round is made that IS legal in NJ. A lot of plinking rounds are legal (FMJ, and the like). I would be very reluctant about using those in NJ, because a FMJ coming out of a .40 will go through your target and into the innocent behind them. Be very cognizant of who and what is behind what you are shooting at in NJ.