Neither will get you any gas mileage increases. Exhaust will get you some extra top end HP at the cost of low end torque. This means you'll have higher top end potental but might loose some low end pull 0-60. When they start coming out with promised parts, Heads, cams, etc. then you can add power. Exhaust is only good for adding power if it is restricting the system but this isn't a problem with most modern cars. In fact, Civics and other rice rockets actually loose a lot of power when someone bolts on a fart pipe because they don't have the backpressure necessary to make the engine work optimaly. If you want a good note or look, get the exhaust. Intake is worthless untess you like the look. If you add a supercharger and heads, the story changes.
If you want to save gas, accelerate slowly from stop and use cruise control as much as possible. Keep the speed under 74 and you'll save a lot. I improved mine by 4 mpg doing this, vs. fast starts and cruising at 80-85. :excellent