The mileage difference between a 3.5 and a 5.7 is not that great- only 2 mpg. If fact, some drivers of 3.5 Chargers have been complaining about poor gas mileage. Don't buy a car that you will not be satisfied with.
3.5= 19/27 mpg
5.7= 17/25 mpg
If you compare the all-wheel drive models, the mileage is identical at 17/24 mpg.
As an option, you can wait until 2010 when the 3.5 is replaced with the new V6 Phoenix engine.