Brave Ike

Steady breath. It's his exclusive skill, and it is very useful on tank builds for units like Lukas. Aether is available on Chrom, who is a 3-4* unit, once he is upgraded to 5*.

If you have a spare Shiro or 4* BK sitting around, you could technically do both. Just give steady stance 2 to the unit you want to give both skills to, at least daylight/new moon as well, and you can then give Luna/sol, Aether, and steady breath.

If choosing between one or the other because you lack one of the above steady stance fodders, I’d say steady breath.