Pete, just remember that if you go to an independant shop you'll have to pay a Ford dealer to re-flash your tune for the differant gears. Unless you are getting a real tune. I've driven the V6 with the 2.73 & 3.31 and bigger is better. I would like to try the 3.73 but have heard enough stories about noise to make that mod unlikely.
I first test drove a 2.73 and would not consider it and got a 3.31 car. After 3 years of hemming and hawing I finally took the plunge into 3.73s and it is one of the best mods I did.
As far as noise, I have the tinyest whine between 55-60 but only when I am on the throttle maintaining speed. I will be the first to admit I am anal about anything being "wrong" on the car and this is minimal enough that it doesn't bother me. It sounds no worse than a little tire to road noise.
Get the right shop to do the job properly and for the right price.