Helical. Feathers are naturally helical in shape and applying them in straight offset won't let them work just right.

I've always used right helical, because most all the stores in my neck of the woods only carry right wing feathers. I've bought a few dozen arrows made up with left wing feathers though, and really liked how they flew. Just not enough better to justify going out and spending the money on a left wing clamp. I'm a tightwad and readily admit it.

Anyway, make your choice on left or right helical based on what's available at your stores. If you intend to order all your supplies from places that carry both left and right wing, then it doesn't matter.