I have an OBS with a ZF-5 42. The factory gears were 4.11 with 33" tires. I have owned the truck for 24 years, about 5 years ago I had 3.55 gears installed, before at 2000 RPM in OD I was at 58 MPH, now at 2000 RPM I am at 70 MPH, but I could no longer start off in second gear. I had to always use first. If I was loaded heavy 10 - 12000 lbs on the trailer and I was on a hill I had to put the transfer case in 4 low. I was not liking this at all, neither was the clutch, I was debating on going to 3.73. I started looking at options and decided to install a ZF-6. Going to the 6 speed is night and day verses the 5 speed. It has made driving the truck a joy again. Empty I can start in 2nd again and even did 3rd once when I was not paying attention.
I say all this to make a comment to my experience. I was hoping to be able to cruse at a higher speed on the highway and get better MPG. I can cruse at a higher speed but there is no MPG improvement. I attribute that to the fact that I am going faster and there is more wind drag, when I am stuck having to travel at 55 - 60 I do see a gain in MPG. I am very pleased with the 3.55 gears and would do it again. But consider the primary use for your truck. I think that when I am loaded I might get better MPG with the 3.73, but I am not willing to spend the money to find out.
Good luck on your decision, When you are done let us know how you like the change.