So in Dragon Ball Heroes is pretty clear, Goku Ultra Instinct is able to overpower Cumber, something Super Saiyan Blue Vegito Kaioken times 50 wasnt able to do. So in Dragon Ball Heroes Ultra Instinct has over 50 times a larger multiplier than Potara Fusion. But how about the canon universe? Are there hints or statements which shows which would have a larger multiplier, Ultra Instinct or Potara fusion?
The Multiplier for Ultra Instinct is definitely greater than the Potara fusion Multiplier. We know this as the SSJ Vegito for the Buu Saga, is nothing in comparison to MUI Goku. There are several other examples indicating the same:
- SSJG Goku being stronger than Vegito in the Buu Saga(MUI is several times stronger).
- UI Omen completely overwhelming SSJ2 Kefla and MUI is even more stronger.
Your real question seems to be-how does Ultra Instinct work in comparison to Potara?
Potara combine the max power of the fused and then multiply it several times.
Ultra instinct base form-was weaker than Jiren in normal form. Each Jiren power up after that was definitely implied to be a multiple of his base form. Now, once UI was mastered, the increase in power from base form to strong enough to beat super maxed out jiren-would have had to be a multiplied enough to overtake him.
So potara is x+x = 2x * 2(or more) = Total
UI is X * X ^ X
UI would be a greater upgrade, the farther Goku reached into it