straight offset fletching creates less spin, but also less air drag
helical creates more spin and more air drag

out of the bow the speed will be the same.....

more spin = better downrange performance
mare air drag = worse downrange performance

with one or the other it will make very little difference because they both have charecteristics which are better and worse than the other. I have shot both out of my bow with no change to sight pins out to 80 yds.

Chris