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.