The steeper (higher) the gear the quicker the car accelerates. Also the higher you go the shorter (quicker) the lower gears will be, getting above 4.10's in a manual and I say you don't even need a first gear just start in second.
Generally manual drivers go with 3.73 and autos will install 4.10s but that is not a rule, it depends on what you want to do and the effects of the gears.
Gas mileage becomes affected by a few MPGs (usually drops) but this also depends on city/highway driving and more importantly the driver's habits.
Make sure to get them installed at a reputable shop and done right with warranty, assuming you're an amateur mechanic like most of us.