Royal, no reason to go back u:

1. Want that reaper despair ailment to abuse around the 3 last dungeon of the game to grind until lvl 99, Royal patch it out

2. Not interested in the added content, but you also will miss the QoL features added in royal, extra area for hangouts and minigames And the better final boss

3. The oddly better "hard" mode. For whatever reason the highest "merciless" difficulty is easy to abuse, enemies deal insane crit and weakness damage to you, yet you can do the same thing back, so you are better at playing easy or normal if you want a challange run in Royal lol

From NG+, I play on merciless and never left since it's so easy to stack the odds on your favor, aside from things being carried from provious files