Hello, friends. Today I’d like to discuss the single most important problem facing Planet Earth at this very moment: Gogeta vs Vegetto. And/or the Fusion Dance vs Potara. Let’s look at the evidence. All the evidence. In chronological order. Should take like 5, 10 minutes tops.
First, Elder Kaioshin introduces the Potara by saying their effect is greater than the Fusion Dance (効果はフュージョン以上). He offers no further clarification. Viz translates this as “it’ll work even better”. 
It’s only later that he mentions how the Potara are permanent, so apparently this is a different aspect than the “greater effect”, unless he figures he needs to really spell things out for the dumb-dumbs. It certainly takes Kaibito by surprise.
(One might question exactly how knowledgeable Elder Kaioshin even is when it comes to Metamorian Fusion to make this comparison, but he does seem pretty all-knowing, and has been watching the Gotenks/Buu fight at least.)
Goku later compares using the Potara with Mr. Satan to a power of 1,000 increasing to 1,001, or possibly even getting weaker (Viz’s translation leaves out these numbers, but keeps the idea of adding their strength together).
(Of course, Goku’s only just been introduced to the Potara, so these musings aren’t necessarily an accurate representation of how they work.)
Next, Buu claims that Goku and Vegeta merging (合体) together still wouldn’t be a match for him (Viz turns this into the rather vague “the two together still don’t scare me”). Obviously, this turns out to not be accurate.
Still, some claim Buu is thinking of the Fusion Dance, and that this shows the gap between them. In other words, they say Buu's correct in thinking a hypothetical manga Gogeta couldn’t beat him. I'd just view it as standard villain arrogance, even if he does have Piccolo’s brains
Backing up a bit, I should probably mention how the Fusion Dance has the stronger character suppress their power to match it with the weaker, while the Potara doesn’t require this. Though it's never said this suppressed power remains off-limits during the actual fusion.
And that’s basically it for the manga…which also means that’s it from Toriyama. NOTHING ELSE AFTER THIS POINT IS ATTRIBUTED TO TORIYAMA. Write that down somewhere.
Next up is Weekly Jump No.28 for 1995, which claims the Fusion Dance (unlike Potara, implicitly) unifies Goku+Vegeta’s spirit and is therefore able to draw out their power to the max. Accordingly, Vegetto should only win against Gogeta if the fight runs over 30 minutes (it says).
Like I said before, this doesn’t come from Toriyama…though there is a doctored version of the image that makes it look like it does. Instead, Toriyama’s only direct contribution to this piece is an earlier part about how Gogeta’s clothes are Metamorian garb.
Also, nothing in the manga suggests the Fusion Dance is better at unifying the mind/drawing out power than the Potara. Though I guess it does make a certain sense that doing a coordinated dance would unify minds better than just putting on earrings.
Next up, Daizenshuu 4’s entry on the Potara (1995). This basically just repeats Elder Kaioshin’s vague “like Fusion, but the effect is greater” statement, with no further clarification. The lack of a time limit is again seemingly separate from the “greater effect”.
Then we get Daizenshuu 7’s Potara entry (1996). This one flat-out says a Potara fusion has greater power than a Fusion Dance one. There’s no vague “greater effect” line here, so it seems whoever wrote this interpreted “greater effect” as “moar power” and made that explicit.
Breaking chronological order for a bit, the Chouzenshuu 4 Potara entry (2013) is pretty much just a reprint of the Daizenshuu 7 one, and therefore says precisely the same thing about Potara giving more power than Fusion Dance. See the above tweet for a translation.
Back to the 90s, the second volume of the GT Perfect Files (1997) describes SS4 Gogeta as having power “tens of times” (数十倍) greater than a single SS4. That’s a description that’s going to come up again.
In fact, for that matter Toei’s official GT site says precisely the same thing about SS4 Gogeta, with precisely the same picture too. toei-anim.co.jp/tv/dragongt/ch…
In 2004, to mark the release of the game DBZ3/Budokai 3, V-Jump ran a feature on the three movie characters included in the game, and gave them all “estimated power levels”. Gogeta was 2.5 billion, Broly was 1.4 billion, and Cooler was 470 million. Hooray?
(I suppose there's nothing in this bit that's directly relevant to the main topic at hand, but it does seem to get brought up a lot.)
In 2009, the manga Super Exciting Guide described the power-up from the Potara as so incredible it’s like multiplying the two characters’ power levels together. The caption even helpfully reads “Goku x Vegeta = Vegetto”.
However, the SEG also describes the Fusion Dance as multiplying the two people’s power together.
Jumping all the way into Super, in ep.114 (2017), Vados describes the Potara as increasing Caulifla/Kale’s power by "tens of times” (数十倍), which you may recall is the same description earlier given to SS4 Gogeta compared with a regular SS4.
Then this very year (2018), the Dragon Ball Super Warrior Sticker Wafers Z (ultimately the only real source of canon) put out cards pegging Vegetto Blue as 9300 and Gogeta Blue as 9600. Truly evidence that demands a verdict.
Finally, the latest Weekly Jump has a DBS: Broly promo piece describing Gogeta and Vegetto as “equally-matched ultimate trump cards” (いずれ劣らぬ最強最後の切札だ). If you’re a master of origami, you might be able to twist that into some shape that doesn’t spell Gogeta=Vegetto.
I wish I had some sort of final conclusion, but it really comes down to what you do or do not opt to view as canon. Even the original manga isn’t super specific on this issue.
