Not even close to the same car. They are based on the same platform, but the FX, Altima, Maxima, M and G37 are all on that platform, too. Hardly makes them the same car. The Infiniti is a nicer car. The interior is better appointed and finished. There is a lot more attention paid to deadening the interior from road and engine noise.
All that said, if you're looking for economy, go with an Altima or Maxima. Better mileage from the 2.5 version of either than the G35 and you don't have to run 91 octane.