2009 Audi A4 2.0T vs 2009 Infiniti G37?

The G37 is still quite frugal even having a 3.7L V6. Obviously the Audi is more frugal but the G37 has the performance as you stated. The Lexus IS350 is also another worthy decision. Also consider that the A4 you're looking at is FWD compared to the Infiniti's RWD set-up. The other alternative is Mercedes-Benz' C350 Sport which is a little bit more expensive but packs a heck of a lot more kit and has a more than able 3.5L V6 and RWD. If you go for top-spec G37you also get AWD which is more balanced and capable on rough roads than both RWD and FWD. This is around 500 dollars cheaper than the C350. The Infiniti will have the cheapest maintenance costs but all of them will be relatively expensive to maintain. Personally I'd go with the clier Merc but if fuel consumption is an issue the G37 gets 18/25mpg compared to the Audi's 23/30mpg, and the Merc gets 14/25mpg which gives the A4 the obvious advantage in that area but simply because it has a smaller engine. The G37 would still be the best option followed by the Merc and then the Audi.