Atk Smoke or Swift Sparrow

Both of those skills are great, it really boils down to who you want to inherit it on. Atk Smoke and Swift Sparrow are both one of the best C and A skills in the game. However, imo Atk Smoke is more valuable because only B!Lyn has it at moment and you basically run that skill on most units. There is a Atk Smoke SS, but you would be forgoing other good SS such as Speed +3 and Attack +3.

I'd say save her for swift sparrow definitely. Atk Smoke is a sacred seal anyway, so it's not really as special as swift sparrow is.