User blog comment:Mich, Michele & Kuhla/Future Units Preview/@comment-26575441-20160115053731/@comment-26336535-20160116034209

Rahgan's SBB increases in damage as you use it, so it deals a lot of damage on it's own. Then he adds Ignore DEF. Lucina's ATK boost is better though, unless you already have one or you have Ultor, Korzan, or Owen as subs already (they have higher ATK buffs). She also adds elements. It's really just based on preference here.

Any unit with HoT basically replaces Selena as a sub unit, unless you need her angel idol buff.