As others have pointed out, torque is a force, while horsepower is force able to move something at some rate. If you lean against a brick wall without moving it, you're applying force, but you're not generating any power at all. If you push on a box and move it, you're generating some power. The faster you move it, the more power you're generating.
An extreme vehicle example (i.e., Teslas and diesel-electric locomotives) involves electric motors, which generate their peak torque at stall, when they're not making any power.
The conversion between torque and horsepower at a given rpm is straightforward: hp=torque*rpm/5252.1, or torque=hp*5252.1/rpm
So now we can talk some actual numbers, instead of various claims which may or may not be true.
Using this, we can see that the Ecoboost, which makes its peak torque of 420 ft-lb at 2,500 rpm, will be making 25% more horsepower at 2,500rpm than the Ecodiesel does at 2,000 rpm, where it is making its peak torque of 420 ft-lb (convenient for this comparison).
If both them manage to have perfectly flat torque curves from 2,000rpm to 2,500rpm (they don't, but lets just say they do), then they are both producing 420 ft-lb at 2,000rpm and 420 ft-lb at 2,500 rpm, and they're also producing exactly the same horsepower at the same rpm, within that range. Despite the much higher peak power, the Ecoboost advantage isn't as much as you might have thought in this commonly used range.
Of course, the Ecoboost produces less than 420 ft-lb at 2,000 rpm, so, at 2,000 rpm, it's also producing less hp than the Ecodiesel at 2,000 rpm.
But, at 2,500 rpm, the Ecodiesel is producing less than 420 ft-lb, so by the time you get to 2,500 rpm, the Ecodiesel is producing less hp than the Ecoboost.
Somewhere in the range of 2,000 to 2,500, the torque curves cross, but they're still not too far apart.
The Ecodiesel produces about 160hp at 2,000rpm. If that's enough for the task at hand, great. After that, torque starts to fall off, but rpm is rising faster than torque is decreasing in our little formula up above, so peak power for the Ecodiesel arrives at 3,600rpm. After that, torque falls faster than rpm goes up. Time to shift up to the next gear.
The Ecoboost produces about 200hp at 2,500rpm. Torque falls off after 2,500, but not as fast as rpm increases, and peak power arrives at 5,000 rpm, even though it's only making 383 ft-lb of torque by then.
Now, don't get me wrong, 383 ft-lb is still pretty good, and the Ecoboost can generally haul a given load (within the acceptable range for the truck) up a hill faster than the Ecodiesel if the driver puts his or her foot in it. It may, however, have to turn more rpms to do the job, and it will certainly use more fuel.
