The world of shonen is full of characters striving to be "the strongest", as well as a myriad of famous swordsmen, demonstrating their unique and impressive techniques to the awe of audiences everywhere. Amongst the scores of swordsmen, The Legend of the Northern Blade's protagonist Jin Mu Won only pursues revenge for his fallen father, never once aspiring to be the greatest; despite this, he is still in the upper realms of power, speed, and endurance.
Similarly, One Piece's Roronoa Zoro didn't initially grow up wanting to be the greatest swordsman in the world but instead took up the dream in the name of his childhood friend Kuina. As a result, turning into a veritable beast and one of the most popular anime swordsmen around. It's only natural then, to compare these different warriors and ponder who would triumph if they were to cross over into the same universe.
The Strength Required to be the Greatest
Growing up in the ruins of the Northern Heavenly Sect under the constant watch of the Central Heavenly Alliance, Jin Mu Won secretly learned the "Gathering of Ten Thousand Shadows", the hidden martial art of the Northern Heavenly Sect's founder Buk Jin-Hu. This powerful set of swordsmanship techniques forms the basis of Jin's strength, allowing him to blow away all the first generation disciples of the Gong-Dong monastery with just the air pressure from unsheathing his blade. He followed this up by defeating the head of the monastery Mu-Gwang with a technique called "Shadowed Blade of the End: Spirit of the Fallen Star", which creates a giant torrent of shadow energy above Jin that he can use to attack his enemies after exerting overwhelming mental pressure upon them. This technique is comparable in power and style to that of Zoro's "dragon twister", without requiring Jin to take to the air in order to utilize it.
With his mastery of the three sword style, and after training during the time-skip with One Piece's current strongest swordsman "Dracule Mihawk", Zoro's biggest feat in the series came when he split Pica in half (after he'd assimilated himself into a mountain) using the "billion-fold world trichiliocosm". In chapter 69 of Legend of the Northern Blade Jin achieved a similar feat, when channeling all the shadow energy he could muster into a technique called "One Focus", he dissected Jo Cheon-Woo and simultaneously sliced the entirety of a mountain range behind Jo in half. Jin is also able to use "internal shadow" to detect and pinpoint the location of any living presence in a large surrounding area, the same way that Zoro was able to use observation haki to find Pica's real body inside the mountain. Accordingly, Zoro and Jin seem on even footing with regards to their physical power output.
Cursed Blades Carry their Weight
The origin of Zoro's many swords throughout One Piece has been explored extensively throughout the series' 25 years of serialization, focused on high-quality blades of legendary status and/or cursed blades that are powerful with their own personalities. The recent notable addition to his arsenal is "Enma", the former blade of the legendary swordsman Kozuki Oden, which is capable of destroying an entire cliffside with a light swing but has a temperamental personality, drawing all of Zoro's haki out of him forcefully if he's not careful.
Jin Mu Won's blade is called "Snow Flower", a "bewitched" (cursed) blade of his own creation, which took him 2 years to forge from a fallen meteorite. The bewitched personality of Snow Flower comes from the female leader of a tribal group that formerly worshipped the meteorite, but who were subsequently slaughtered for a demon blood art ritual, imbuing the leader's desire for revenge into the stone and subsequently, into Jin's sword. Unlike Zoro's cursed sword that tends to rebel against his wishes, Snow Flower has been repeatedly shown to assist Jin in combat. The spirit of the blade was able to possess "Chung-In" when he came into contact with it, preventing him from stealing the sword whilst Jin was asleep. The spirit of Snow Flower has also appeared in a ghostly form around the blade, restraining Jo Cheon-Woo's arms in his fight with Jin, and has also willed Jin's body forcefully when his consciousness was fading. Bearing this in mind, Jin's weapon of choice seems to give him a slight advantage as things stand.
Beyond the Limits of Human Endurance
Jin Mu-Won has endured a lot of physical hardships throughout the series to date, including but not limited to withstanding Jo Cheon-Woo's "Fists of Shining Heavens: Rain of Heavenly Destruction" which carved massive chasms into the landscape, blocking the sword god Yeon Cheon-Hwa's "Illuminating Sword Outburst" and surviving his final "Phantom Blade" technique. Despite the impressive nature of these feats on the surface, in reality, Jin Mu-Won has often been shown to receive medical assistance and has also had the rely on Snow Flower to maintain consciousness in some cases.
By comparison, Zoro has one of the most famous endurance feats in all shonen with the famous "nothing happened" scene, where he took on all of his Luffy's pain from the fight against Gecko Moria in Thriller Bark expecting it to kill him. He was then found by Sanji the next day, surrounded by his own blood, but still conscious and standing tall. More recently, Zoro survived a joint attack from two emperors of the sea (Big Mom and Kaido), which hit him head-on. Therefore Zoro has the advantage in endurance and in an extended battle, would likely be able to soak up more damage, until his opponent ran out of stamina.
Ultimately, a fight between Zoro and Jin Mu-Won would clearly be a tightly contested battle, taking everything the two swordsmen have in order for one of them to prevail. With Zoro having a small advantage in a battle of endurance, and the pair evenly matched in terms of strength, the battle will likely be decided by the combatant's use of their respective blades and how the blades react to the fight. Snow Flower's influence could be a deciding factor in the fight for Jin, but at the same time, if Zoro were to overcome Enma's current rebellious nature it would certainly offer more in terms of offensive power and turn the tables on Jin.