I want to warn you right now.. the V10 is NOT a hot rod motor... its a TQ monster and really comes into its own when you put a load on it. If you test drive a 99-04 ish super duty Crew with the 5.4 and then turn around and test drive an identical V10 model you may not be able to tell a whole lot of difference with the trucks empty.
05-10 Super Duty have a nice upgrade in power and transmission (4 vs 5 speed) You will be able to tell a difference between the 5.4 and the V10 empty in these trucks.
However, with all that said.... the 5.4 is a GREAT motor and does a great job up to about 7500-10, 000 lbs depending on the gearing.
IMHO.. I would not pass up a great deal on a 5.4 Super Duty Crew Cab, however a V10 is preferred for towing.
I have had both and in my experience both motors are great. However, my 5.4 experience is in an F150 with 3.55 gears.
If you get a 5.4 or a V10 go for the 4.10 or the 4.30 gears. The 3.73 gears are not a great match for these motors in the Super Duty.
I know this reply is getting long. I apologize for that.
I would also strongly consider the early Dodge Mega Cab 1500 (has 3/4 ton axles, wheels, and frame) and will come with a nice Hemi V8. And later the Mega Cab came in the 2500 and 3500 series. Both had the Hemi available. This is a great motor and a GREAT cab choice for families.
The GM 6.0 or 8.1 are great too. If you go newer you will have to get a 6.0 if you go gas. I don't know what year they stopped making the 8.1. Maybe 2005 ish? The only issue that I have with the GM Crew Cabs.. at least in the early 2000's is that they seem to have a bit less leg room in the back compared to the Super Duty and Mega Cab. I have this same complaint on the Ram with the short crew cab (club cab?)
Thanks and good luck!
Jeremiah