For Voidwatch 90 Amano already beat Masamune. Inside Abyssea Masamune will still have an advantage.
Depends on the mob and your strat. One Masa SAM/THF did enough damage to take down all of the Tier 4s while everyone else focused on Procs.
The biggest advantage to Masa is that Fudo scales with TP. SAM will almost never WS at 100tp and has JAs the guarentee 200tp WSs. Add that to tp bonus moonshade.