The torque just isn't in the same ballpark. My V10 planted me in my seat too off the line, and driving around with just the truck I was NEVER at a loss for power. But towing relies so much more on the torque curve which is far different in the diesels. With the V10 while towing, it takes longer to get to it's peak torque and even then it's less than the diesel.
With that said, the V10 pulled a trailer and vehicle just fine, both flat and the hills around my house. But when towing a load, it doesn't feel nearly the same as when you're romping around with the truck by itself.
This was my experience stock vs stock: The V10 will beat up on the CTD while driving around w/o trailer. Add a trailer and the the V10 will fall on it's face far more than the CTD, and the diesel will be much more responsive while pulling. But in the end, both will tow a rig on a trailer.
Of course, once you add a tuner, the diesel wins no contest in both situations.