Fusion techniques have been crucial a part of Dragon Ball lore ever since their introduction in the Buu Saga of DBZ. This overpowered and almost too-convenient ability is primarily used as a last resort.
However, there was a time when Fusion was so prevalent that there wound up being multiple methods to perform it with. Let's take a look at the Potara Earrings and the Fusion Dance, how they work, and which one could be stronger.
The Potara Earrings
The Potara Earrings are worn by Supreme Kai, and when a single one is worn on opposing ears by two people, they are immediately fused together -- even if they don't want to be. If a Supreme Kai is involved, the Fusion lasts forever. Otherwise, it only lasts for one hour. However, without a Kai, if the fused warrior's body can't sustain its power, the Fusion can split at any time. This technique combines the participants' power levels, then multiplies it tens of times.
Vegito, the most famous user of the Potara Earrings, is the Fusion of Goku and Vegeta. He has appeared twice -- once to fight Majin Buu, then later to battle Merged Zamasu -- though he failed to defeat either of his opponents. Against Buu, he opted to put off destroying the monster until he'd retrieved the people who'd been absorbed, unexpectedly splitting once inside Buu's body. In the case of Merged Zamasu, there was little Vegito could do against the immortal god, despite being the stronger fighter. Goku and Vegeta appear to be too powerful to Fuse with Potara in Dragon Ball Super, as Vegito split after just 10 minutes.
Merged Zamasu is the result of the Fusion between Goku Black and Zamasu. Due to Zamasu's status as a Kai, this Fusion is permanent. Additionally, the villain is immortal thanks to Zamasu's wish on the Super Dragon Balls. Along with Goku Black's Saiyan abilities, the Fused god was incredibly powerful. However, like Vegito, Merged Zamasu's body couldn't withstand the strain of his power, but because he couldn't separate, his body became deformed and his power drove him mad.
Kefla, the Fusion of two Universe Six Saiyans, Kale and Caulifla, made her debut in The Tournament of Power. Champa, their universe's God of Destruction, gave the women Potara Earrings to use as a last resort. Her power feels almost limitless to other fighters, and she easily overwhelms Goku in his Super Saiyan Blue form. Though she was eventually eliminated by his imperfect Ultra Instinct state, Kefla's power had her feeling like she could've blown away an entire universe. Interestingly, Kefla is the only example of a Potara Fusion whose body had no issue maintaining her power.
The Fusion Dance
The Fusion Dance, a technique created by the Metamoran race, is a more complex method of Fusion. Using a precise series of movements while in perfect sync allows two people to join together. The participants must be completely equal, from their power level to their breathing. The slightest mistake can result in a deformed and weakened warrior. This transformation, like Potara, combines the two power levels and significantly multiplies them.
Gogeta, another Fusion of Goku and Vegeta, has a slightly more extensive history than his Potara counterpart. He has appeared to take on Janemba, Omega Shenron, and Broly in various canon and non-canon battles. Despite disposing of Janemba in 77 seconds, he spent a good 10 minutes goofing off against Omega Shenron before his Fusion abruptly ended. All business against Broly, Gogeta was ready to deliver the final blow before Shenron whisked The Legendary Super Saiyan away to safety.
Gotenks, the most frequently used of all Fusions, is comprised of Goten and Trunks. He's stunningly powerful despite being so young, managing to achieve Super Saiyan 3 simply by seeing Goku transform once. At one point, Gotenks even tears a hole through dimensions with his voice. While using Super Saiyan 3 he is roughly equal to, if not slightly stronger than, Super Buu. Gotenks is also the only recurring Fusion who's never failed to reach his maximum time limit.
Potara Earrings Vs. Fusion Dance: Which is Stronger?
Considering that each Fusion technique has nearly identical power boosts, it doesn't seem like one of them could be stronger. However, the superior technique in Dragon Ball is clear. Potara Earrings, despite lasting longer, have a history of instability when used by powerful beings. This could possibly be because they are intended for use by Kai, who are far from warriors.
The Fusion Dance, on the other hand, is much more reliable considering the Fused warriors never struggle to sustain their power. Only Super Saiyan 4 Gogeta has ever failed to last the full 30 minutes, but this was because he was actually too weak to stay Fused thanks to the savage beating Goku had just been politely handed by Omega Shenron.