I had 3.55s in my '07, and now have 3.73s in my '11. For city driving, the 3.73s are way more fun! If you don't have a good tire in the back, you will have traction issues if your right foot is a bit heavy. I can spin the tires in 1st with just hard, roll-on acceleration.
Regarding highway mileage, I have found this to be true for me:
'07 GT, 3.55, 5-speed stick, 70mph, 2,250 rpm = close to, if not...30 mpg. Strange, I know.
'11 GT, 3.75, 6-speed stick, 70mph, 2,250 rpm = at best, 22 mpg, MAYBE 24...but it took forever to register.
I have no idea why the huge difference in MPG if both cars turned the same rpm on the highway. Both had
K&N drop-in air filters. Stock tires. Windows up, AC on or off.
If you like to DRIVE, you'll have much more fun with 3.73s. The rpms are not crazy at all on the highway (2,250 at 70 in 6th). Around town I drive my car like a 5-speed and with my VERY short commute of under 3 miles one way, I get between 11.5-12.5 mpg. That's not driving like a crazy person either. Half the commute is 35mph or less. If I use 6th gear around town (pfthhh), I loaf along around 1,200-1,500 rpm and can see 15mpg or so.
I just read that you have a V6, so ignore my MPGs totally.
Hope that helps.