The faster you can get your arrow spinning, the more stable it will be in adverse conditions, especially with broadheads attached. In addition a shaft that has a tendency to plane slightly, will plane far less. Always go with the maximum amount that your rest allows.

Helical is preferable over offset whenever possible. A helical clamp will only fit on an arrow shaft properly, one way. Therefore, there is no adjustment range. Large diameter shafts end up with more helical than small diameter shafts and 5" fletches end up with more than 4".