I've got a 99 Cobra with 4.10s. Other guys are right, probably not much difference so might as well go with 4.10s. Only thing that I've noticed is how much more gas I go through due to the higher RPMs. At 60 mph I'm cruisin at about 2200. But if you honestly bought this car to get great gas mileage, you need to go ahead and sell.
I have 4:10's but with 27.1" OD 255/50R17 tires on the rear that actually makes it a 3.89. Did that on purpose as I really wanted the 3.89 but hard to find. I get 24 mpg at 70mph 2500 RPM. The taller rear tires with 1" spacers really give the car a nice rake with the 275/40R17 (25.7" OD) on the front. Plenty of pull in all gears.