Well if you only plan on using your powerstroke to haul your date to the prom, the 3.08 gears may just work for you! Remember you are talking about the same gears that come stock in a Mustang that only weighs less than half what your truck weighs and most people like to swap them out to at least a 3.55 for performance reasons.
If you do any towing at all be it "light" or not, I would not go any further than a 3.73 in a truck. Most F-150s that have 3.55s are dogs and can't hardly get out of their own way. I have the 3.73 with 20" tires and when you compute the difference in tire height it brings my effective gearing real close to that of a 3.55. That being said, it is not the ideal setup for towing. Unless you have spent a few bucks on mods, the six oh has quite a few more ponies than a 99 7.3 and I'm telling you that a 3.55 is less than desirable with the extra power.
The bottom line is that it's your rig and you need to set it up to suit you, but if it were me getting ready to drop that kind of dough for a gear swap I would talk to a few people with that setup. It would be nice to be able to drive a rig with the same setup first.