The 2006 G6 (GTP) has possible axle ratio's of:
3.29
3.55
3.69

Meanwhile the G6 Sedan (2.4L) offers a 3.91 ratio.

What is the real world driving difference among the ratio's?
Is it worth getting a model with a higher or lower ratio?

Thanks.

The final drive ratio is going to help your off the line acceleration. Since these cars are not top end performers by any means.. the 3.69 ratio is the best bet. It may lower mpg on the highway compared to the other gears (Maybe 1.5mpg) but will give the car an advantage taking off.

It also limits top speed so to speak..but these cars will never see that limitation so no worry. :)

The 2006 G6 (GTP) has possible axle ratio's of:
3.29
3.55
3.69

Meanwhile the G6 Sedan (2.4L) offers a 3.91 ratio.

What is the real world driving difference among the ratio's?
Is it worth getting a model with a higher or lower ratio?

Thanks.
The ratio is meaningless, because...

The GT (3.5L) has the 3.29 only.
The GTP (3.9L) has the 3.69 for the auto, or 3.55 for the manual.
The 4 cyl has 3.91 only.
The base 6 cyl (non-GT) has 3.05 only.

So it's not like you'd pick the model just to get some specific drive ratio. You pick the model for equipment or option levels, or in the case of the GTP, for transmission preference.

Someone isn't going to say "I'm not getting the 6 speed which is what I want because it's only a 3.55 ratio, so instead I'll get the auto which I don't want so I can get the 3.69 ratio."

Basically, you can't choose ratios, so they're a moot point.