Both have insta-win moves in theory, but both tend to lead with CQC. Let's go over the insta-win moves first.
Darkspine has reality warping/text manipulation on a universal scale with just a hand gesture while Excalibur has durability nullification with his sword that pierces defenses. Base form Sonic has evidence of being able to resist the former but not the latter, which is a point in Excalibur's favor.
In all fairness, it seems like a bit of a NLF to say that Excalibur can cut through Darkspine effortlessly with so little evidence to go off of, so let's assume both fighters resist each other's instant wincon. That means both will rely on thier go-to close quarter combat skills, especially since they can both reflect each other's projectiles. This means that Excalibur has a range advantage with his extendable sword and a skill advantage via being a more level headed fighter who isn't literally fueled by pure rage. On the other hand, Darkspine has qualitatively higher lifting strenght and elemental manipulation to make up for not having a weapon. Since lifting strenght is only a meaningful advantage if he can close the gap between them, and base form Sonic already has a ridiculous resistance to heat, Excalibur's advantages are better than Darkspine's.
Voting for the knight of the wind. To think he would have been shit-stomped to oblivion just a year ago...