Fuegan Searsong lance or beam

Hi, considering whether when within 6" vs a single monster/vehicle the Searsong lance might be outdone by beam if also having hit with Lhykhis that turn so critting on 5s, along with the Peternatural precision for lethal hits (as an aside does the sustained hits 1 stack with the existing sustained hits 2 for beam if selected?). Would be great to have confirmed for a T 9-12 target if possible. A few more complications, he would be deployed from a Falcon with the Fire support active and in an Aspect Host for reroll 1 on the hit roll.

I'm guessing cover and saving throws vs the attacks non melta component might be the deciding factor?