Maybe I've missed this in the above but the 'difference' between WF and DT lines is pretty straight forward .. as is the calcualtion of the grain weights (assuming the manufacturer doesn't 'lie').
The 'Maths,' as the Brits would say, really is pretty simple and I'll try to keep it that way as I'm doing the numbers off the top of my head at 0315. (Yes, my work day starts at that time of the morning.)
Soooo, the line (first 30' feet) is 300 grains (lets skip what the forward/back taper does to the number, I have no clue). So that's 10 grains per foot. With a weight forward line all you've got is the '300.' With a Double Taper you keep getting more as you pull off more line. Each additional 12 inches is another 10 grains. 60 foot of line in the air that initial 300 is now 600.
You no longer have a 'x' rated line you have one close to double that number (again, I'm not going to look up the exact).
Which is the better line, for what combo of reasons? Your call.