what is best to use regular unleaded or premium fuel?

I would use premium in everything if you dont mind the extra cost. But regular is fine. Fourstrokes are not very finicky with fuel compared to two strokes.

In reality lower fuel makes more heat and heat is power. Run the lower octane when you can especially at elevation. Air is thinner and unless you have monster compression you won't see detenation problems.

Only use premo if you have a high compression engine or you can hear it knocking.

The only other time would be is if the premo is 100% gas (no ethanol) and the regular was a "blend" and I was going to store it for an extended period. The blends will separate and make starting harder.