Tachi: Fudo is probably SAM's best WS, though Kaiten with a 90 Amano should beat it in some weird situations.

Kaiten (Amano 90) - 3.75 fTP, 60% STR
Fudo (Masa 90) - 3.65 fTP, 60% STR

Fudo is going to win most of the time though, because Masa 90 has 15 STR on it and Amano has.... 35 Acc. Only time Kaiten will be better than Fudo on average is when 15 STR (and the resulting ~12 Attack) fails to make up a .1 fTP difference.

In Abyssea it doesn't stack up, but outside of Abyssea the Empyreans are pretty balanced (ignoring ODD aftermath issues).