The Sinner vs Zeldris is in all honesty a tricky match:
- If we go by statements, then the Sinner should be way above Zeldris in power. Zeldris himself already stated that Cusack and Chandler were individually stronger than him, and there's no reason to think he was lying, so it's logical that if they fuse the difference will be way bigger.
- If we go by feats however, Zeldris has shown to be way more impressive than the Sinner. The last one was being kept at bay by a Rhitta Charged Night Escanor who could stop his blows and tank multiple hits, overwhelmed by King using only one spear from far away, humiliated by Ludoshel until Crisis started powering him up and oneshotted by Mael even after being powered up multiple times. Zeldris, one the other hand, trashed the entire Assault Squad by himself with ON, kept up with King's spear from far away with only his 1st DM and finally kept up with Mael and tanked multiple attacks from him (he also blitzed and injured him considerably). Not to mention that the DK's magic makes him almost invulnerable to magical attacks and ON it's incredibly difficult to bypass with physical attacks. It's undeniable that Zeldris is far more impressive than the Sinner.
This fight, as well as other examples, show one of my main problems with Nakaba's writing and that is contradictions between feats and statements. Both sides can make up an argument as to who wins this battle, but I'd personally say that Zeldris wins. Statements are nice information and are make up for a reason but at the end of the day they're only words. The feat is what backs up the statement, if a character A states that a character B is way stronger than him and then this same character A shows feats that are way more impressive than those shown by character B, I will believe character A is stronger at the end of the day.
Just my two cents on the matter.