I think we're talking about the same cottonwood, it does make nice planks when it's dry. A rough sawn cottonwood plank on a scaffold will be lighter and stiffer than southern yellow pine or hem/fir. The cottonwood will be 2"+ and the others will be finished 1 1/2". The stiffness is from the thickness. Cottonwood will also be tougher because it's a hardwood (though softer) and the unique grain of cottonwood, vs Ash that would like to split like Douglas fir. That's why I'd like to say Elm, it will stand up to a beating better than Oak, Ash, Hickory, but it's not rot resistant like white oak or black locust.