I'm thinking that Vegetto / Gogeta Base should only be > Vegeta and Goku PSSJB in the same Arc.
I think god forms at least rival fusions the most, since as someone said, neither Vegetto nor Gogeta could use the UI, Mega Instinct, since both forms contradict each other.
I think the fusion multipliers vary depending on the Arc.
I think in Arc Moro, definitely Vegetto/Gogeta PSSJB would be much more powerful than Moro OG73-I, but not astronomically stronger. It would not be strange that Moro with all the skills and magic he has, can hit Vegetto / Gogeta PSSJB and manage to divide it. I think I have Vegetto/Gogeta PSSJB Arc Moro, a bit more powerful than UI Goku (Arc Moro).
In the Arc Granolah, I don't have multipliers from UI and Mega Instinct forms over non-God forms. Vegeta when strengthening in his Mega Instinct form, does not strengthen his SSJ forms. In the Arc Granolah at no time did they even think about the possibility of merging, despite the fact that in the end, Goku and Vegeta teamed up. I have a hypothetical Vegetto SSJBE (Chapter 87) stronger than True UI Goku and Vegeta Mega Instinct (Peak), but not by a very big difference.
At least I think that always Vegetto / Gogeta Base are more powerful than the PSSJB of the same Arc, but nothing more, since lately Vegeta SSJBE has rivaled the god forms of Goku of the same Arc.