You definately do not want straight, it's pointless unless you concerned with rest clearance.

You want either helical or offset. Helical wraps around the arrow shaft, offsets sits on the shaft at an offset angle...

I prefer Offset clamps on a fletch jig like the Bitzenberger because of the versatility. Helical will get you slightly more spin quicker out of the bow however an offset clamp allows you to regulate how much or how little offset you can use. If your strictly a drop away rest shooter, extreme helical is a great clamp because your going to get optimal arrow flight because of spin.

Spin is the key to good arrow flight out of a well tuned bow and matched tuned arrows, particularly for broadhead stabilization.