Have it checked for any leaks. Also maybe your air filter or spark plugs need changing. These 2 are common mpg destroyers if they are dirty or worn out.
Probably new due to the fact it will come with a warranty from the dealer. The parts for BMW are very expensive and its nice to have a warranty with your fresh BMW and not worrying about part and labor cost.
Used. Once you drive off the dealership with your new vehicle, the value goes down faster than you can say "Infiniti". If you know your cars, know how to check for everything (engine, transmission, timing belts e.t.c.) then you can find a great deal on used Infiniti's. That's how I found mine.