Hero Island
For those who start to lack ambers, like i do… while xp/amber is best for the lv 100 gear (if you **dont** have to fuse), i believe lv 40 gear is best xp wise, since every time you fuse ambers, you lose half of them. This will require a lot more hearts, but i suspect majority is stuck on ambers, not hearts right now.
This only matters of course if your smithing level isnt max yet. If it is, fuse and go for lv 100 only.
BREAKDOWN
Once you can craft lv40 gear
lv1: always fuse stones, only craft until you unlock lv20 crafting
lv20: always fuse stones, only craft until you unlock lv40 crafting
lv40: never fuse stones, always craft (best xp)
lv60: craft when you earned the stones without fusing previous stones
lv80: craft when you earned the stones without fusing previous stones
lv100: craft when you earned the stones without fusing previous stones
This will require a lot of hearts, but we always had a surplus of those and never enough ambers in past Hero Islands. Once you have max smithing level, you can fuse it all and craft whatever you want or whatever you need for lv100 gear