First of all, you won't even notice the difference if you only switch to 3.55. You MIGHT notice a little bit, but it wouldn't be worth it.
Second, the 3.73 is a perfect gear if you ask me. Despite what the guy above me thinks, you won't suffer a terrible gas mileage loss. You may even pick up a few MPGs around town since you will be able to use 5th more instead of leaving it in 4th. A 4.10 gear could even be used if you wanted. There are plenty of people that run 4.10s on the highway.
Toy: 2002 Mustang GT
Daily Driver: 2010 Ford Fusion Sport