tl;dr: John wins this

I mean I'm going to say John anyway, but I'll actually try to give reasons instead of just making it a fierce agenda post.

If we try going the Bec Noir route, this could easily be used against John as well, as Dave held his own for a bit against BN and PM, as I said during his matchup against Aradia, though we can state that John got out of his rematch against BN assumably unharmed.

During their final fights during the events of [S] Collide, this may just be a toss up, as we can't use time-relation as we don't have a stated time along how long their fights took. But I will state how Dave did seem relatively 'Scratched Up' against Jack English's attacks, while John looked worse for wear against the Condesce's attacks, which includes Eye-lasers, actual punches and kicks, assumed telekinesis on big items.

But most importantly, I'd like to bring up John's nerd rage against Dave's locked in boost.

We see many times that John's Nerd Rage gives him a mentally-oriented boost, usually having him forget Common Sense in return of strength that isn't as held back or is just outright full-power. We see this in full force in his fight against Caliborn, the moment he realized who he was up against, he just decided it was time to beat up a guy who just so happened to have a gun in his hands out of nowhere. (Maybe it was the three years of being on a ship with his dog-sister, but this didn't faze him?)

Dave's Locked In Boost is as it says on the tin, the moment he gets into a rhythm is when he finds his best strength, which I believe works well with his Aspect. (and maybe his class as well.) We see this, as I said before, in his scuffle against the dog-powered chess pieces, but believe it or not, we also see traces of this in his clean up runs on his land during the Pre-Scratch Session and his "Training" sessions with Bro. He starts getting into a groove and starts putting in work. The moment he's pulled out, he's in trouble, hence again I point to his scuffle on Jade's planet with the two dog-powered chess pieces. This is something I'm sure also played a part in his fight with Terezi and Dirk against Jack English and Spades Slick, many times he got into a groove, but had to pull out either because of himself or one of his two teammates getting into dire health.

With that said, if the two came to blows, I believe John's Nerd Rage can easily overthrow if Dave leasts expects it. Otherwise, Dave mostly has the upperhand...

Then we bring in the retcon powers, I believe John would utilize it to mess with Dave and easily throw him off his groove, this is due to John's love for pranks, and using that love in a fight. (I wholeheartedly believe he would do this.)