I've had both the 7.3 and the 6.8. For towing, I'd go with the 7.3. Like others have said, the V10 will do a very good job hauling your trailer, but it's not the same as a diesel.
As far as milage, my 7.3 would get around 16 mpg empty, and 12-13 towing
my 9k 5th wheel. My 6.8 gets around 13 mpg empty, and 10 towing my 7k
TT. I live in the mountains, and am finding that my V10 does not pull the hills as 'effortlessly' as the 7.3 would.
I was actually looking for another 2002-2003 7.3 Crew cab, when I came across a great deal on the V10 I ended up with.
It really boils down to what you want and what you are willing to spend.

 

In my humble opinion go with the 7.3 1999 and up were all good (air to air aftercooling) if maintained properly......lots of power and torque and accept performance chips for super pulling power if desired.....as far as mpg depends how you drive and how heavy you are, I have witnessed these trucks make 18 pulling BUT not at 65mp/hr...same old story slow down and save a buck!