Hi,
I have both trucks, I just recently purchased the SRT10 and have not been able to fully race it. If you begin the race rolling, you will leave him behind, at least at the beginning, depending on his mods. But if you begin in a dead stop, he will beat you on 4x4 high.
Mine is a 2009 FX4 with the banned version of H&S, Gearhead tunes, Volant Cai, IC tubing, AFE exhaust, DPF removed, EGR removed and BMWheels. It will beat almost anything on a dead stop. When doing so, the truck launches so hard that I alway loose my right food from the accelerator and have to reaccelerate again, and my wallet, which is always on the roof console ends up in the back seat. A tuned diesel truck will smoke too bad on WOT. If that truck smokes, beware of a dead stop race.
It is also very powerful when you are driving on highway at around 70mph, I have left behind Bmw serie 5, mod Leon Cupras, Mustangs, Mercedes and volvos.
The 6.4 is the diesel engine from which you can get the most power out of it. It is on the Powerstrokes from 2008 - 2010 and the reason among others, is that they have 2 turbos. All of the other powerstrokes only have 1.