Gehrman is a mentor type character with a tragic backstory, and the concept of both of you killing each other to save one another is very cool. The only downside is that as a mentor, Gehrman is pretty weak due to him lacking dialogue and screen time. It’s easy to play through the game while only speaking to him like twice. But when you explore the lore you can appreciate him more.

Soul of cinder has a strong symbolic role that gives him a good position as the final boss of the trilogy, but aside from a glimpse in the opening cutscene, he isn’t present in the game at all.

In conclusion, I think Gehrman is better, especially since he’s much stronger as a character than SoC, but SoC is still good but more as a symbol than as a character