It depends. Do you need to use the backseat? The G37 has a much roomier back seat. Do you need lots of power? If so I would rule out the IS250, as it only has like 204 HP.
Overall I'd probably go with the Infiniti because you can get the equipment and power (actually, it's more powerful) of an IS350 for about the price of the IS250. The interior is roomier in the G, and overall the car is a lot sportier, arguably as good of a handler as the BMW 3-Series.