None really. Obviously 3.55's are going to give you slightly better mileage on perfectly flat terrain given the same speeds. My brothers 5 spd 94 GT gets identical mileage as my 5 spd 91 GT and he has 3.55's and i have 3.73's.
There is about 250 rpm difference...so if cruising at higher speeds is a normal occurance then the 3.55's are better choice. If i were to do it over again i would go with 3.55's for the extra few highway revs. I do notice my mileage falls off a little if i drive faster than 75-80. But my brother can get the same mileage as me at 85!! thats the only difference.
my mileage actually increased by 2 mpg when I installed a 3.45 geared rearend instead of the factory 2.73. I think that had something to do with running the E-303 cam and the rpm's being too low with the factory gears, it was bogging.
Don't let the anecdotal evidence fool you. Few people with 3.73 gears get their speedometers/odometers to be accurate after the gear swap. The error shows more miles driven than what's true, therefore the MPG calculation is off. My buddy's '89 LX has 3.73s and the correct speedometer gear. It still has a 4% error in it. I drove it cross-country for him, and consistently showed 22 mpg based on the odometer mileage. When the 4% speedometer error is factored in, it was really getting just a hair over 21 mpg (still pretty good).
The change from 3.55s to 3.73 won't make a huge difference. With so many other factors that can affect mileage (fuel quality, air temperature, elevation, tire pressures, etc.) , you might not even notice it in day-to-day driving.
Typically you can expect to lose about 1 mpg on the highway for each jump you make in gearing. The engine is turning over more RPMs and that translates to more fuel burned.
Your city mileage probably won't change much (depending on your driving habits). Since the engine has to work less to accelerate the car from a stop, you may even see slightly better city mileage than with the 3.55s.