Seems like a distinction without a difference. I mean a Z era Gogeta may not be as strong as Vegito going by the potata > metamoran rule, but should still be more than enough to handle Gotenks.
I mean, it was stated in Z that Potara gives a "rival boost", where Metamoran does not. So back then the clear intent is that Potara > Metamoran to *some* degree(at least where a Goku/Vegeta fusion is concerned)... We just don't know how *much* stronger Potara was intended to be.
That said, SSJ Vegetto >>>>>> Boohan >> Bootenks >> Super Boo ~ SSJ3 Gotenks. IOW, SSJ Gogeta could still be quite a bit weaker than Vegetto, but still be FAR more powerful than Gotenks.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
And if we're including anime-exclusive scenes like that, then it's worth mentioning how Buu Saga Vegeta fought a copy of SSJ3 Gotenks inside Buu with great success while switching between his Super Saiyan and Base forms
Referring to the Boo Saga, a hypothetical Ssj3 Goten would own Ssj Goku in a fight, so, logically speaking, Ssj3 Gotenks should destroy Ssj Gogeta in the same way.
However, with the latest info regarding how fusions work, it is clear to me that the base power of the fusion output is the combination of the maximum power of the fusees. Then, the fused character can stack up his own transformations on top of it.
So:
- base Gotenks = (Ssj Trunks + Ssj Goten)*unknown multiplier
- base Gogeta = (Ssj3 Goku + Ssj2 Vegeta)*unknown multiplier
Hence why Ssj Gogeta would stomp Ssj3 Gotenks.
Also, we must factor in that Goku/Vegeta fusions appear to be even more powerful than expected.
This approach also explains why Kibitokai was not that powerful, as he was (Supreme Kai + Kibith)*unknown multiplier, where: