yep, it's all about low-end torque when towing, and that's where the diesels and Ecoboost shine. As do the big V-8 and V-10's, but they consume a lot more fuel when not working.
But lots of low-end torque and tow capacity doesn't help when the available truck payload is too low for the trailer's tongue/pin weight. Ford's F150 trucks can have some impressive payload numbers if you get the right options. The Max Tow option helps add 400-500# to GVWR and payload.
But even better is the HD Payload option which can add as much as 1000#, with the improved suspension, rear axle and wheels effectively creating a hybrid truck somewhere between an F150 and F250. Unfortunately it usually has to be special ordered and is not available in all models/configurations.
Dodge RAM is introducing a V6 diesel with even more low end torque than the Ecoboost in their 1500, "half-ton" line this fall. But if they don't improve the payload of their light trucks it may be wasted.