There is truth to what NRG is saying (I wouldn't put it as eloquently as he did, a "joke"---hehe, just razzing you a bit there NRG
), but the MPG argument is a tad overblown. His numbers are almost exactly what I get as well.
It all depends on price, and right now, its a close to a wash between petro and diesel (But the price of gasoline will get up there again in the winter/spring).
If you're trying to move out of your '12 N55 just for the fuel consumption, the deal would have to be pretty incredible to even come close working out for you OP. Of course, I wouldn't presume to know your specific requirements/needs either.
Now the torque argument, you bet, I really appreciate the torque. I'd buy a diesel SUV all day long if it has the kind of performance this one does. I don't see much more performance from the N55 to be honest, but I'm used to the N54 from my other car, so that my have something to do with it.
Good luck in your decision!