Which is more important: SAE rating or viscosity index?

#1
For the purpose of quieter engine and lowering petrol consumption, which is more important: (lower) SAE rating or (higher) viscosity index?

The reason for asking is because while I expect oil with lower SAE rating e.g. 0w20 lower than 0w30, the viscosity index (VI) can be higher in the latter than in the former, indicating that the 0w30 is leaner than 0w20.

So, what's the deal?
 

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