Diameters, the double taper will have a larger diamater throught and will allow it to ride a bit higher in the water than a wf, and will load a bit quicker regardless of how much out of the tip of the rod. you will get probally the same delicate presentation with either, I would mabye consider a 3wt dt vs a 2 if the rod is more moderate than slow and increase the diameter a bit more, 2wt lines regardless of dt or wf are both very small in diameter and will struggle in choppy or broken water to stay up, the bigger the diameter the better if your rod will allow. Its a valid point on why they make them 90ft but the thickness of 2wt running line is thicker than 20lb, 15lb or 12lb backing and creates a larger arbor therefore quicker line retrieve, and I don't know if cutting it down is a good idea and replacing w/more backing you might get core saturation and cause the line to start sinking at an earlier age.