Thanks for the input. I'm still partial to getting a complete rearend with the 3.73's already in place. That way I don't have to play with the gears and preload and all that stuff. Plus I'd like to be able to setup the ECM without having to go to an aftermarket setup. It might be possible to fake out the ECM if you can compensate with a smaller tire circumference.
Five-Oh Brian: Have you driven the 3.73 in the auto or are you just basing it on the fact that you have the 4.10's on yours? Did you go from 3.33 to 4.10? Seems like a huge change.
The factory computer can not be adjusted enough to fix the speedometer after a gear change. Get an aftermarket handheld tuner.
I've had cars with 3.73's, I've driven S197's with 3.73's, etc. It's not enough gear, IMO. My car came from the factory with 3.31's. I had the 4.10's installed when the car was 2 weeks old. I had 4.30's in my '03 Mach 1, 4.10's in my '93 T-Bird 5.0L LX, 3.73's in my '89 Mustang 5.0L LX, 3.90's in my '68 Pontiac GTO, etc. All of these cars had much lower-numbered gears initially, then I chose steeper gears.
Search the forums for a consensus and you'll find some people who installed 3.73's in their 2005+ Mustangs who are now wondering if they should have done 4.10's or 4.30's. You will likely never (or very rarely) hear anyone with 4.10's or 4.30's wonder if 3.73's would have been better.
3.31's/3.55's aren't enough gear, 3.73's are OK, 4.10's rock, and 4.30's are a bit too much for some, and 4.56's are overkill for 95% of the Mustang owners out there.
Fuel economy doesn't really drop too much on the highway due to the wonderful overdrive transmissions we have. Fuel economy in town is basically unchanged with steeper gears, though, as the car accelerates easier with less throttle position than with the lame 3.31 or 3.55's which lug the engine from a stop (or low speed). On the flip side, drivability and pure enjoyment is increased ten-fold with steeper gears, and that's why so many people have switched to 4.10's.