To be fair. Astra's activation rate isn't much better. 24% isn't all that better than 12%. It's double, sure, but those numbers are both really low.
If we're doing this comparison on a situation where both activate, Lethality is instantly better.
Lethality kills instantly. The only thing (to my knowledge) that can block a Lethality strike is Miracle activating.
Astra can miss, be affected by Pavise and Aegis, and triggers Counter. Although, to be fair, it CAN crit. But Lethality can too. And for Lethality, it doesn't matter.
In the case where Mirace activates, Astra is better... If it doesn't activate on the 5th attack. If it DOES, then neither Lethality nor Astra have an advantage in that situation.
But, on topic:
It bugs me how Swordsmasters have two swords in their field sprite, but it's not that big of a deal. They look cool.