You will get a few different opinions on this one as this topic has been debated for years and years to come....here's my thoughts
I have had both the 3.55's and 3.73's in my 87 GT with a T-5 and a Tremec 5 speed.
I myself prefer the 3.73's with my set up, the car likes to run at a little higher RPM and the 3.73's are just right. Now my car is a fair weather car and just a toy, so gas mileage and highway RPM is not a concern to me.
For a naturally aspirated car I like the 3.73's, on a supercharged car I like the 3.55's. An AOD I like the 4.10's....