Alright, to bring a conclusion to the Diantha vs Red argument I bring you the definitive answer:
A video using a 5/5 Diantha with Devastation Sync Nuke
https://streamable.com/c3xpt8A video using a 4/5 Res with Haymaker + Sun
https://streamable.com/vl3f6oBoth of them 6EX, both against the same enemies and both using relatively the same Parameters
>Physical Dmg barrier>Special Dmg Barrier>No Status Conditions>Sync Buff ^5>Ally Healing v5>Power on Hit 1>Gradual Healing>Sync Countdown -2>Power on Hit 2>Ally Move Gauge - 2Red got to pick No Stat Reduction and Str +1, Diantha had to pick Str +2 instead.
If you disagree with team compositions, grids, stat distribution, or playstyle on either video you're free to reply with a video recording of your own performing better.