I use helical flethcing now all the time but:
For all practical purposes, for example: shots 15 - 25 yrds, I've found , if my bow is tuned to the arrows I'm shooting the helical and straight don't really differ much. But shoot longer ranges like 30 yrds and up the spinning arrow is much more stable, and hits the target at a better angle hence penetrates better. Also at very short range, the helically fletched stabilizes faster than does straight fletched arrows. The straight fletched arrows wobble a bit in the first 10 yards, stable out and then begin to wobble again as they loose inertia further into flight. It's like a smooth bore musket vs. a muzzle loader rifle, both firing a 400 gr buffalo bullet, at close range there's not much difference, but at 75 yrds there's a world of difference, the spinning bullet makes a much nicer hole.