Reiri and Kirstin are both hentai-retarded, but Kirstin is way stronger.
She uses an oversized gunblade, can stack a ton of buffs, has access to way more equipment, can 1-turn wipe crowds of enemies, and isn't afraid to walk around in the nude. Reiri has a normal sword for most of the game, can stack some all right buffs, is dedicated to fighting 1v1 and taking her sweet fucking time with it, and the equipment she has access to is generally less broken than Kirstin's.
Kirstin's out here slaughtering super-demons in the ruins of an ancient empire in her post-game content (and can do so while pregnant), Reiri still needs to watch her stamina bar when fighting half the enemies in the game.
Personally think Reiri's hotter though. Actually replayed WDK a little while ago and it made me wish Dragon Reconquista had more content like it; Reiri walking around in the nude and getting harassed/assaulted by enemies or NPCs, some playable sections where Suzurika (Sword Saint) and Merumeia (Merchant + Staff Frenemy) could be ambushed/knocked down by monsters, or any of them getting "picked up" by NPCs in certain locations. Doubt any of that'll ever happen, but both games were plenty fun in their own way.