User blog comment:XCaidx/Hisui vs Rain as sub/@comment-29807989-20161205194231

For one side Hisui has: No. His SP option for BB fill is specific to guarding. Guard your units a turn and they could get up to 8BC per hit. Where as Rain's BC bump is a fill. 8BC fill for all team members for 3 turns once his BB is triggered.
 * 8 BC fill on use (SP) (BB/SBB).

Personally, I like BB gain on hit (granted from other units) better than Hisui's BB when guarding SP option.

I was running Hisui lead for a long while, but run Rain lead now paired with Rina. But you're asking for a sub reccomendation. . and for that I guess I would say, do you have a tri-stat buffer covered for the team or not? If not, sub-rain > than Hisui.

As subs, you're really just wanting their 50% 2 turn mitigations I'd guess, so to me the difference maker is the tri-stat buff option from Rain.

My $.02

HTH