There's way too much overthinking these kinds of "this or that" topics. Kendatsuba/+1 has significantly more evasion and magic evasion, have a huge critical hit rate boost (which benefits impetus a lot) and a ton of TA. Herculean and relic/artifact are basically minor tp increases, but damage on monk is hardly ever going to be your problem. If all you ever care about is damage, the choice is simple, but it's never ONLY about damage.
I've said it before, when in doubt between two close pieces of gear, go with the one with the higher defensive stats. That will aid your overall damage much more than any attack on a single piece would.