Regular stay set is easier to handle and actually cheaper. X is too stiff for sheets but great if not perfect for halyards. That extra stretch on a jib
sheet won't matter, but the poor handling will drive you nuts. The parallel core
in X is what makes it so stiff. Just perfect for a halyard
under a constant load. Stronger is not always better.
I would think something 7/16 or 1/2 would handle well in the hand and on a winch
. 3/8 may be strong enough but may not work so well on a self tailing winch
. I would also consider Marlowbraid if you come by it for a good price
. I like the feel of it and it is a bit stronger than ordinary double braid.