https://www.bg-wiki.com/ffxi/The_Mog...upower_Effects
Artisan's Advantage: The chances of losing ingredients due to failed synthesis will be significantly lowered!
  • Players must receive synthesis image support from their respective guilds in order to benefit from this Super Kupower.
hm, seems you're right, guess I didn't need signet; only synth support. I thought that one required signet & synth support.
Regardless, can't be in an Ionis area and get Artisan's Advantage buff.
As for which is better, no idea, but Ionis is basically always getting its success + mat.loss buff, where the kupopower is more rare so I typically like to think the kupopower mat.loss is better than Ionis' buff.

If the kupopower isn't around I always synth in an adoulin zone with ionis for anything expensive. For cheap stuff like breaking avatarites it's either ToAU or WotG areas for their food duration buff for me.
Currently 7 areas controlled on my server in WotG so that's +160% duration. get a lot of mindless crafting done in that time.