Not a lot of difference in those ratios. Pick the 3.73 if you tow and
the 3.55 if you don't (or not often or heavy).

The 3.55 will give you a bit better gas mileage. Really depends on what you
hope to gain by changing gear ratios.
jor

Depends A LOT on tire size.  With the modern 4 speed autos, and the common
usage of larger tires as standard equipment, I'd be more inclined toward the
3.73.  Remember that the Triton engines (4.6, 5.4, and V-10) are OHC designs
and prefer more RPM than the old pushrod engines.

CJB