There' been entire volumes written onthis subject, but I'll try to summarize it.

It all depends on the size and loading percentage of breaking strength. If you're trying to maintain minimum diameter then wire is the way to go. If you're trying to save weight synthetic is the route.

Amsteel (sk75) is not appropriate for a luff wire unless you have a way to retension it - too much creep

Vectran has low creep, but has very low chafe and UV resistance - good for halyards, but not good for running or standing rigging.

AS-78 is an improved version of SK75, has excellent chafe and UV resistance, but still exhibits creep when loading at 60%+ of breaking strength.

Technora has low creep characteristics but absorbs moisture causing the fibers to swell. Once the fibers start swelling, it creates internal chafe within the braid and starts breaking down. You won't see it until the rope breaks, and it looks hollow on the inside.

There's a relatively new product out - Dynex Dux 75. They take SK75, heat and draw it through a die process. It realigns the chrystalyne (sp?) structure resulting in it being 40% stronger than SK75 and only having 3% creep at 90% load. 7mm Dux 75 weighs only 3.4 kg per 100 meters, and has a breaking strength 15,000 lbs.

Its pretty cool stuff, I've seen it used on ORMA 60's and Volvo 70's for standing rigging, strops, etc. I've used it to create backstays, inner forestays, and halyards for big monos.

Currently, I'm in the process of replacing all of the standing rigging on a R33 to reduce the weight aloft.