excellent informantion and advice in this thread so far....but if I may, I'd like to put in a word for the 3.55s.
1.) 3.55 car felt noticably faster than the 3.31 car that I initially I test drove. Obviously, the 3.73s are a bigger jump and will feel even faster "seat o' the pants" but the 3.55s are no slouch and you will notice them. Regardless of how the gears feel, it's conceivable that the 3.55s are faster than the 3.73s in some situations.
2.) I have plans to campaign this car in Autocross. The 3.55s are supposedly more useful in this application since 1st and 2nd will take you up a little higher in the speed range. I have heard from a few road racers that this is their gear of choice as well but this will probably vary from track to track/driver to driver.
3.) Even with some power adders, you can finish the 1/4 mile in 4th gear rather than having to shift one more time into 5th. Early on, it was speculated that the 3.73s were chosen to optimize the 0-60 and the 3.55s were chosen to optimize the 1/4 mile...and so far, it's hard to tell if that's right since all the tests are done on 3.73 cars.
4.) More versatility by changing tire sizes- more gear- go a bit smaller, less gear, go a bit taller. The 3.55s put you right in the middle and give you a pick...but that's admittedly a stretch.