User blog comment:B214/Question and Answers 6/@comment-118.172.180.221-20160719145800/@comment-24.113.252.213-20160719161025

Definitely Sirzechs.

Although my reason isn't something as simple as a "power gap" between the two, but a matter of experience and how their abilities interact.

Sirzechs has hundreds of years of experience over Issei, so even if Sirzechs did have a power disadvantage (which he doesn't), it would likely play out the same way it did between Issei and Cao Cao in their first two battles (Cao Cao dominated the battle). Power (Issei) vs Technique (Sirzechs).

Sirzechs possesses the Power of Destruction. We've seen that the ability can easily destroy Issei's Scale Mail (at least parts of it). This was shown when Millicas (someone less powerful than Issei) used his Power of Destruction to destroy Issei's shoulder armour. Granted, that was Issei's BB armour, so that's not the most accurate example to follow. But I'm sure you get my point.

It would be close if this fight was just about Power, but Sirzechs would definitely win due to his experience and superior technique.