Ah.. now this is an interesting one.
Killer might scale slightly above him, for hurting Kaidou with his Dura Neg technique, and iirc reacting to the Emperors and Roof-Top Supernovas, as well as taking attacks from them. Guernica was hurt badly by Drake and Apoo even though he did pull through and defeat them.
I guess I'll vote Killer.
Interesting take.
Now, outside of Killer's dura neg, Guernica actually has the AP advantage, via badly damaging Zoan/Hybrid Drake (who off-screen took hits from Apoo's DF) and badly damaging Buso-defense Apoo, although the gap isn't that large.
Speed-wise, they're about the same, as Killer could dodge Kaido's blows and keep up with Early Wano/Rooftop Zoro, while Guernica blitzed Apoo (who reacted to Zoro) and managed to reach Luffy before Kaido could react/finish swinging his club down, although Guernica can use Soru to boost his speed greatly, so he takes the speed category.
For context, his Soru blitzed Zoan X Drake, who is as fast as his base form who reacted to Base Yamato, so his Soru will blitz Killer.
Durability is equal, as both took hits from yonko-level heavy hitters (Killer took an Indra and later ate a Fulgora from Big Mom, while Guernica ate a Kazenbo and then shook off a Boom way better than Post-Udon Base Luffy did). But while Guernica can amp his durability with Tekkai, it doesn't mean much when Killer, as you mentioned, has those broken-ass durability negating Sonic Scythes.
This is an incredibly close one, and will come down to who can strike the fatal blow first... I mean, Guernica can blitz
hard with Soru, but can he take Killer out before Killer unleashes his Sonic Scythes?
I honestly don't know at the moment, since it seems like it could go either way, so Imma wait for more people to input.
Killer:
+ Sonic Scythe go brr
= Strength
= Speed (against Base Guernica)
= Durability (Against Base Guernica)
Guernica:
+ Tekkai gives him the durability advantage
= Strength
+ Soru gives him the speed advantage
+ Rankyaku (which he definitely has, since all CP9 members must master all 6 Rokushiki arts and CP0 >>> CP9) gives him the range advantage