I too wanted the 5.4 when I was looking for trucks, but at the end of 2008, we got a deal on my 06 with the 4.6l that we couldnt pass up (i could sell it today for more than I paid for it 3 years ago). I dont tow ever, but I haul a fair amount and drive it on 800+ mile road trips 4-5 times a year. I have never felt like i didn't have the power i needed. The mileage is good, 18-19 on the highway and its a 4x4 with a 3.73 rear end. It isnt fast, but if you put your foot in it it will chug to 90+ to pass truckers on 2 lane highway.
In closing, you will be happy either way, unless you plan on towing, if thats in the plans, get the 5.4.